I'm not sure if you change them in NifSkope, I believe you can do it in the GECK. Please don't take my word for it. Goddamn it if I had the game and the GECK installed I could show you how in a heartbeat. Maybe I can explain it to you.
Please understand it's been a very long time since I've done this (2 years I think)
Open up the GECK, find the weapons you want to edit and then when you select it opens up a window with all of the info and shit about it. When you're in the weapon properties screen, go to the art and sound tab. To the right of that tab is where you can choose sounds. There are a few for attack sounds, should be one for reload, dry fire, equip, etc. However you have to add them in, I think you can do it there w/o adding the sounds in the audio section.
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Nicki is my Mom but Tay is my Queen