It's not really feasible.
If you are trying to use sound waves, the maximum volume you can attain is 194dB. The reason for that is physics. 194dB corresponds to a sound pressure level of roughly 1 atmosphere. This means that, on the negative side of the wave, the sound pressure just barely hits vacuum.
If you attempt to make any louder sound, you are forcing the air to operate in a non-elastic manner, so we can't talk about "sound waves" anymore. The wave equations assumed an elastic medium. Now what you are talking about is a sonic boom.
I've actually looked for how much damage can be done with a sonic boom, and it's not much. You can break windows, which are nice and rigid, but other than that, it's almost impossible to actually cause any real damage. You're certainly not going to hurt anything which is remotely armored.
Which puts lethal sonic weapons in a funny position. There is no way to kill with sound which is not made immediately better by using the same hardware to push a bullet downrange. Bullets are simply better at conveying momentum and energy.
Or alternatively, only use them on a planet consisting entirely of glass people and glass buildings.