Okay, just keep in mind that altering speeches is not possible (so far), gta-ivaud doesn't support opening speech files. So if u'd like to give niko a different voice for example.. thats not an option.
Altering gun sounds etc is possible, when looking at weapon.ivaud u'll see there's more then 1 file per weapon. It might help downloading a similar soundmod to the one your planning to do just to see which sound files need to be replace. But the newest ivaud version has named soundfiles and u can preview them so I'd say you'll find the right ones. Good luck