I do notice that even with a totally empty Playlist, if you turn Music "on", CanonGuitar.sgt will play.
See item 2B in my writeup.
Dang, just noticed you can do it within the Music Playlist screen, too.
Just choose "add" and then navigate to:
(Win XP)
C:\Documents and Settings\"your account name"\Application Data\Marine Aquarium 3\
(Vista & Win 7)
C:\Users\"youraccountname"\AppData\Roaming\Marine Aquarium 3\
and it shows up "CanonGuitar.sgt" just like any other supported music file.
Much easier.
It would be if that file existed. But it doesn't get created with recent versions. Not exactly sure when that happened, but if you have that file, it's a "leftover" from some previous version.