You don't need FNVedit for that. Find the sound you want, convert it to the .wav sound format and make a second copy of it. Go into your games data directory, find the sound folder then go into the fx folder inside it then find the obj folder inside that folder, then lately open up the stealth boy folder. There should be two files called obj_stealthboy_activate_01 and another one with a 02 at the end. Rename both of the wav files you made to those Stealthboy names and either overwrite the ones in the folder or Delete them, and replace them with your two wav files.

It should now be to whatever you changed the sound too.

Sorry if this is a bit messy to understand or any misspellings, I'm currently typing this with one hand because i nearly cut half my thumb off earlier (I'm fine now)

@TheRageMan

The mod author said that it was designed to work like the Chinese stealth armour, is it actually possible to attach a sound effect to a piece of armour and here the sound every time you go into sneak mode ?

Oh so you're wanting a sound effect to play when you crouch. its possible, but Ive never really used FNVedit for sound stuff. I believe you would have to use the GECK Kit for that considering it would most likely require an ESP