Tl/Dr: Music permits the AIs to tap the human spirit during combat. This helps them work with the humans rather than merely commanding them. Music has an advantage over other communication media because it can be woven deeply into the heart of what makes us human, and over thousands of years, can instill a true warrior in all of us. This makes it safer to have around for generation after generation than other forms of communication.
War is hell.
I like to start a lot of my combat answers with that. It's an important reminder of what we're discussing. War doesn't leave room for comforts and laziness. If you're not on top of your game, you may die when you could have lived. One of the challenges facing any warrior entering the combat zone is how to stay on top of their game for the duration of the encounter. Communication is one key to that puzzle, and it's the one we'll look at here.
Obviously the AI's are going to need a language to communicate with. Traditional orders like "attack," "retreat," or "hold this position" are valuable in combat. However, these are only single commands. If you're leveraging an AI like this, you're going to be getting a stream of updates. Streaming updates benefit strongly from context. As you receive orders, the way you interpret the next ones changes. This lets you be more efficient with your instructions. Instead of constantly issuing perfectly objective orders like "do not let any enemy combatants get within 50m of this position until the air wing drops their ordinance," you can simply say "hold here," with a context of a) you've been told to come to 'here' already and b) you've heard the orders being delivered to call for close air support. The difference between the 19 word command and the 2 word command can be life or death.
With an AI constantly streaming customized commands into your ear (or into your mind), this could be taken even further. The cognitive load of receiving orders could be nearly nothing! There is, however, a catch. You have to train the humans to understand this stream of commands, and that could be very difficult. One reason the military relies on drilling commands is to ensure that in the chaotic horror of combat, orders still get acted out. You can only drill so much into a cadet over a few years. For them, simpler is better, and they have spent hundreds of years optimizing the exact best way to train cadets to follow orders in combat with the training time they have available.
Anything worth doing is worth doing slow.
What if you could train your soldiers from infancy? Imagine the warriors you could create. Imagine how expressive of a combat language you could instill in them. If you had that much time, you could even push the language into the subconscious, permitting it access to the much faster reactions that keep us alive (the human brain can react within a few milliseconds, but conscious thought reacts no faster than 300ms). Instead of just issuing orders directly, you might organize the mind of the combatant like a regiment itself, with cascading orders which reach down to the fine little feedback loops that permit us to walk smoothly while chewing bubblegum.
Of course, this would not be a fun society. You can imagine how much trouble we would get in if every one of us was indoctrinated from birth in an art of war. We'd tear ourselves apart. This is one of the fundamental challenges of a military. How do you remain combat ready instincts when there is no fight to be had? For that, we turn to the Eastern martial arts.
A student said to his master: "You teach me fighting but you talk about peace. How do you reconcile the two?" The master replied: "It is better to be a warrior in a garden than a gardener in a war."
The East teaches us to focus on harmony with one's world, creating a difficult challenge for the warrior. How can you be harmonious with a core that is ready to tear the world asunder if need be? The approach I see most resonant in the martial arts from the East is a effort to make every facet of their warring art dual purpose. Every piece of their puzzle can be used for health or the betterment of others. You can learn an entire martial art and never once learn to fight, merely how to be a better person.
The secret to this approach is the warrior spirit. The warrior spirit is the part of every fighter that is ready to take all of those health and wellbeing pieces and restructure them back into that of a warrior. If your teacher considers you worthy, they'll give you some of the hints as to how these pieces can be arranged (or you might find some of them yourself). When the warrior spirit stirs, the peaceful monk rises up with all of his instincts already primed and ready.
Humans are a notoriously violent species. Many use this warrior spirit to get their way in civilized life. If we've been partnered with AIs who are strong and stable enough to be our conductors in the battlefield, we may have handed them the keys to the warrior spirit long ago. They may be the keepers of the true human warrior spirit. In our hearts might be a lesser spirit, unaware of the full power of the lessons their society is teaching them. This spirit may be quickened at the right time by an AI into full on war, or might merely be "mostly quickened" into something which can do war so long as it has the AI fueling that spirit, and then die back down after the combat is over. This would be a very powerful adaptation.
But how do you protect the secret? If the evil Computer Overlord were to ever find out that the humans were primed for combat, ready to respond to the instructions of an AI, the overlord would immediately leverage this and destroy humanity. The keys to this ability need to be protected. The solution is to bury this ability deep in the heart of a human. Each human contains a small undeveloped warrior spirit held close to their heart (metaphorical heart). Anything which wants to leverage this warrior spirit will need to have the blessing of the human's heart. Each human is different, so the torture and experimentation on one individual will not necessarily help reveal the secrets of how to harness another.
One purpose of music is to touch the heart of the audience. Now we see why music is such a powerful tool in the hands of our AIs. With an AI conductor fine tuning the subtleties of timing and mood in the music, we may admit their instructions that, if issued as a verbal order, would have to be processed by the mind first. With music, they can awaken the warrior spirit and then being to talk to it directly, instructing it in how to best leverage the body around them.
I gritted my teeth as a round went through my shoulder. Even in a grapple the AI's were more than clever enough to take a shot if one presented itself. Balthazar was good, but we hadn't worked together before. After having been hand picked by Drax to be trained under him, working alongside a different AI just wasn't the same. Balthazar was certainly trying his best, but his song always seemed to be just one beat behind.
Then the song changed. Or perhaps I should say I assume it changed there. When on the battlefield, you never really detect the changes in the AI's conducting. You're too busy trying not to die. None the less, I know the song changed because I suddenly found an opening. I found a resonant frequency in the servos of the robot I was grappling with and was able to apply my full bodyweight against the weakest linkages in the robot's body. It sprung backwards under its own force, unable to adapt to the sudden change in combat so quickly. Then I heard the first distinguishable change in the song -- a slight insistence in the snare drum that I hadn't been hearing. A tap-tap-tap just strong enough to ensure I heard it. A cold fire stirred in me, igniting all that it touched.
"We've got Drax back online!" I heard over the comms. All I could reply was "I know." I had better things to worry about than the comm techie monitoring our channel. I knew he was back on line from the instant I heard his fingerprints on the snare. I didn't want to speak to a techie, I wanted to spend my focus on Drax.
"It's good to see you again," Drax finally said as he added a few French horns in the distance. "I'm sorry I was away, you understand what networking errors can do." Drax's style always amazed me. His control over his conducting was so precise that he could afford to speak in such a long winded fashion. No time was actually being wasted as my body and mind unified along with his song. "Let's do something about that Class-R that put a hole in your shoulder."
I looked around and spied a newspaper that had fluttered to the ground. I strode over to it with a posture that declared "this space is mine," and took up a stance. My fluttering hand slowed to a standstill, pointed dead at the Class-R's CPU. The message was clear. Drax wasn't just looking to win this fight. He was looking to gain from it. This was no longer a fight to not lose, it was a game to see how much could be won. Drax knew this, and I like to think the Class-R knew it too. It knew it was outclassed now. I like to think I saw fear in its eyes, but I know the robots aren't to be anthropomorphized like that. Just a straight forward return on investment calculation scanning across its memory banks.
The Class-R took two steps backwards and a flood of B's rolled in in front of it to act as cannon fodder as the more expensive Class-R retreated. Again, I like to think that they were quivering where they stood, though really it's just the filter-loop that leaves Class-B robots constantly off balance. Their unsteadiness can surprise you if you don't have an AI to remind you of where their balance lies woven into the strict bassoon line that now filled my ears.
Drax and I put away the rifle. No point in wasting ammo here. We drew the sword in one smooth Iaito move too subtle for one of the Class-B's to see coming, slicing its mobilizer off. I perhaps give myself too much credit. I'd heard Drax shifting the music to a style 50 years more recent than the one he'd been using earlier. I just made a guess that the Iaito move would fit and Drax made it beautiful. That's how it always goes. Observe, improvise, and follow. See an opening, wing it, and follow the conductor's lead as they make it good. Its pretty much the only phrase that was drilled into our heads during basic.
The tempo accelerated, just a hair faster than the Class-B's could be ready for, and we went to work.