TL;DR Justify helmets that work for humans with cat ears for a modern army
There has been a few cat species from medieval/fantasy like khajit and mithra and they have taken a few different methods for designing helmets around cat ears. These seem to be a balance between protection and blocking any advanced hearing that such larger ears might offer.
The three paths I see taken are:
Regular helmets/Cat ears not visible: cat ears dissapear or are squished up in the helmet. Gives good protection, but hearing will suffer.
No armour: Cat humans tend to be lightly armoured and agile so the helmet is just skipped. Good hearing, visibility of the cat ears, but no protection.
Cat ear shaped helmet: These seem to be the best balance for fantasy. They offer the protection of regular helmets and don't crush the ears while still giving hearing ability (mildy restricted by leather or ear holes in steel).
For a fantasy environment I feel the cat ear shaped helmet works best (others have their situations) and I can even see some pretty good examples from the khajit in ESO. There is even some parrallels to horns on viking helmets.
For a modern/sci fi environment I feel the balance is shifted.
Most real world modern helmets only cover the top of the head which is where most of a cat human's ears are. They also tend to have a very uniform shape to help protect from shrapnel and, if lucky, bullets. I have seen some bike helmets that integrate cat like ears but I feel as if these odd shapes would prove a structural weak point compared to the uniform shape of a regular helmet. They also tend to be rather large and clunky, which could limit/slow movement for a much smaller benefit (compared to fantasy/medieval armour).
The same three options that were available for any fantasy army are available for a modern army, but I feel as if the balance has shifted towards no headgear. Except when exposed to heavy shrapnel/artillery warfare I can't see the advantages of helmets for a cat eared human.
Is there a way to make helmets in general viable/preferable for cat eared humans?
Assume you are equipping a modern or slightly future army and would have access to modern technology/materials, but also assume you are trying to do it en mass for a reasonable price.