i havent done this in RO2 yet, but I have in ROOST. I'm sure it will be very similar. goto your red orchestra 2 folder in steam and look for the red orchestra.ini (might be ro2.ini? not sure) file. Use the search feature to find "VoiceVolume" (or similiar). It'll probably have a value next to it that looks like 1.000000. Change that to 0.000000 and then save the file.
**edit**
if you use the forums search feature, you should get many results back from people asking this question about ROOST