see this for changing onager sounds
https://forums.totalwar.org/vb/showt...ghlight=engine

You can only change it if you have access to the .evt files in an unpacked animation. I don't think you can do that for the other units. (Or rather I asked before and Wles thought you couldn't https://forums.totalwar.org/vb/showp...99&postcount=8 - he's done a lot on animations so is likely to be right)