LocalSetMusic
From ZDoom Wiki
void LocalSetMusic (str song [, int pattern [, int unused]])
Usage
LocalSetMusic changes the music in the game, but only affects the player who activated the script. If song is a MOD, then pattern specifies the pattern in the song to start playing at; otherwise pattern is ignored. Pattern is optional, so you don't need to specify it if you aren't using a MOD.
Additionally, you may also specify "*" instead of the name of a song, and the level's default music as defined in MAPINFO will be played.
Note that the third parameter is currently defined but not used by ZDoom. It is recommended that you do not specify the third parameter at all (as in the examples below), as this parameter may be used in the future.
Examples
This command changes the currently-playing music to "D_DOOM" for the script activator:
LocalSetMusic("D_DOOM");
This command returns the music track to the default specified in MAPINFO:
LocalSetMusic("*");
This command changes the music to the 2nd pattern of the specified MOD:
LocalSetMusic("MYMOD", 2);

