You could just use the same tag for all of your backing audio, then when any audio with that tag ends, have it begin a new song selected at random.
On audio ended, tag = "bgmusic"
if random(0,20) >= 10
play audio "track1" tag="bgmusic"
play audio "track2" tag="bgmusic"
then you can just keep adding to the list for each audio track that you have.