starttrackslot <volume#> <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. The music for intro/briefing/loading can be accessed by setting volume to 7.
starttrackslot can be used to play music tracks from other volumes.
Commands with an additional "var" suffix take gamevars rather than constants or defined labels for their inputs. As an alternate short form, "varvar" can be dropped from these commands; for example ife serves an an alias for ifvarvare.