From ZDoom Wiki
void SetMusicVolume (fixed volume);
Sets the sound volume of the music currently playing, as a multiplier.
- volume: The volume multiplier to set. This ranges from 0 to 2.0, with values higher than 1.0 only working if the global volume is low enough so that the end result is not more than 1.0.
This script fades the current music out, and when it is finished fading it, it changes the music and fades that music in.