THobson ins correct. Here's some examples:
0 fadeSound 0
Kills the sound instantly.
0 fadeSound 1
Makes the sound from the game (gun shots et cetera) come back again.
5 fadeMusic 0
fades the music down to zero (the second number) in five (the first number) seconds.
ThereĆ's a bug in the OFP game engine, so it is not possible to fade sounds. They are either on or off, like a lightswitch. Even though the comref says it's possible.