0 fadeSound 0
instantly reduces the volume of the game sound to zero. When you want to switch it back on again you use:
0 fadeSound 1
The first number is the time in seconds before the volume reaches the level you specify with the second number. Volumes goes from 0 (nothing) to 1 (maximum volume).
But fadesound doesn't really allow actual fading in/out of the sound. It works like a light switch. Sound is either on or off. Any number higher than 0 will give you full volume (at least with the sound cards I've tried).