no idea how to actually do what you want, that's pretty deep in the game mechanics, but if this is for some kind of RPG where there wont be many available units (for something with many different unit types this would be pretty painful to implicate) you could use this workaround:
create a custom soundset (aka rewrite one of the game ones) where each type of sound also has a few alternate sounds (walk2,pissed2 etc' )that have blank sounds that you upload, or just pretty unnoticeable sounds from the game. this will lower the amount of times you hear the units talking.