You take the REZ Xtract and you extract the sounds you want to inject later. Go to the folder that was just extracts and look for the guns you want to replace sounds with. Drag that folder out and then you use REZ Inject to inject them back in. When you try to inject them back in, it will say something about duplicate, so you click the bar right of it and you should click the folder you want to put the new sounds in. After you're done press Inject, go into your game folder then drag the old one as a backup somewhere and erase the "New" part out of the new REZ. Easy