Starttrack
starttrack <track#>
starttrack 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. Change volume_number take another episode music. Remember that you must take back the last volume_number value:
getuserdef[THISACTOR].volume_number TEMP setuserdef[THISACTOR].volume_number 1 starttrack setuserdef[THISACTOR].volume_number TEMP
This will start track from episode 2 level 1 at any episode.