First off, I'm new to modding Fallout, though I've played around with the GECK and have a little familiarity with it (i.e. I've made a few custom weapons with what's already there). That said, hopefully what I'm wanting to do isn't too terribly complex (or impossible, for that matter). I'm making a custom weapon and I want it to have a certain sound effect that isn't in the game. Now, I've found the sound and it's currently in .mp3 form. What do I need to do to or with the sound effect file to get it so that it appears in the "Attack Sound (3D/Dist/2D)" menus?

I believe it has to be 16bit pcm(wav) files otherwise it'll crash. Mono for weapons
Once you have that you can stick it in \sound\fx\ then point your audio sound effect to it

Thanks for responding, though I could use a bit more help if you can spare it. I've converted it to a .wav file and put it in my data\sound\fx\ folder, though I can't find it when I click on the "Select TESSound" button next to "Attack Sound (3D)" and look through the list. Do I have to use any particular naming conventions or is there anything I need to do to import it? Thanks for all your help.

Ok, scratch that. I figured out how to get the sound effect into the GECK, but now I can't figure out where I'm supposed to put it. It doesn't seem to want to play when I actually fire the weapon. =/

Anyone know how I can get this to work? I converted the .mp3 file into a .wav file with Switch Sound File Converter. Then I dragged and dropped the .wav file into the audio > sound menu in the Object Window in the G.E.C.K. and it'll play normally when I select the sound for "Attack Sound (3D/2D/3D Dist)" but when I go in-game to test it, there's no sound at all when I fire the weapon. I know I'm doing something wrong, I just don't know what. =/ Any help at all would be greatly appreciated.