Real Life Examples
In WW2 the Nazis experimented with a sonic weapon that used pulse-jet style detonation chambers and resonance to create extremeley loud soundwaves. If two of these units were set up and focused on an overlapping point the sound compression waves generated could, if given enough exposure, kill. That is, if you managed to get anybody to hang out in a very specific area right in between two of them for about 30 minutes. Also each was about the size of a house. Pretty much totally useless as weapons.
In the late 50's a plane was built by the US government, the XF-84 Thunderscreech. It posessed a super-sonic prop, which when turning at full speed produced a rippling series of sonic booms from the tips of each propeller. The resonance in certain hangar structures or nearby aircraft caused damage to them. There are even reports of personel working in enclosed spaces with it suffering detached retinas and siezures.
Problems with this as a biological process
In our atmosphere sound has a top speed due to the ability of our atmosphere to conduct sound waves. In our atmosphere sound travels at about 343 meters per second, dependant upon altitude, humidity, and temperature. This means no sound on earth, at sea level, can exceed about 194 decibels. To produce this sound you must generate compression waves by rapidly oscillating between a vacume and +2 atmospheres of pressurization. No small feat. To give you an idea of just how loud that is and how difficult it is to hit the maximum decibel level, When Krakatoa erupted and was heard around that hemisphere, it was "only" 172 decibels.
Also, "sound" doesn't push things. Waves of compressed air do, the sound is just a byproduct. So your critters dont have to be loud, they need to be able to compress air and direct it at a target. We're at this point more talking about generating explosions rather than sound. I do not feasibly see a living creature under currently accepted theories of biology being capable of doing this naturally. The kinds of forces required are going to be harful or lethal to anybody/thing immiediatley nearby not to mention the creature emitting them, thus not likley biologically generated in origin. Anything generating forces like that is going to be exposed to them too.
I do not believe there is a realistically feasible method to achieve these kinds of forces biologically. Sound can be used in other ways, like to irritate a target and convince it to move, but not to generate the kinds of forces you are looking for without also causing an equal (but probably greater) amount of damage to the creature generating them.
Doesn't mean its not feasible
Instead of physically shoving them with compression waves, your creatures use the sound they generate to modify herd animal behavior. Lets say they can emit a very loud, very well focused "beam" of sound. There are riot control devices made by a company called LRAD that produce extremeley loud, very tightly focused sound beams, (up to 30° beams.) These can cause anything from nausea to irritating pain in rioters ears, and can be aimed at individuals if close enough.
Typically, it is so loud, and uncomfortable, the targeted person or persons will attempt to move away from the source of the sound produced by this device. I'd say that maybe your creatures can use this to aim the sound admitted towards one side of the animal, causing it to instinctivley move away from it in a more desired direction. In this manner they manage to herd thier animals in desired directions of travel.