Starttrack
starttrack <track#>
This command is used to change the current background music that is playing.
<track#> is literally the track number of the current episode (which can be detected with the gamevar VOLUME) as defined by the music command.
To be able to use another episode's music, change volume_number as in the example below. Remember that you must change it back to its last value:
getuserdef[THISACTOR].volume_number TEMP setuserdef[THISACTOR].volume_number 1 starttrack 0 setuserdef[THISACTOR].volume_number TEMP
This will start the music track from episode 2, level 1.