Game->GetMIDI() currently returns 1 - [strike]7[/strike] 6 if it's a built in MIDI, or >= [strike]8[/strike] 7 if it's a custom MIDI. It returns 0 if there's no music playing.
Game->PlayMIDI() accepts 0 for no music, or >= 1 for custom music. I do not know if you can play built in MIDIs via this command.
Here's a better idea:
Have Game->GetMIDI() return 0 for no music, >= 1 for custom music, and <= -1 for built in music. Have Game->PlayMIDI() accept the same parameters. That way, you can pass Game->GetMIDI() directly to Game->PlayMIDI(), and have it work as expected. (Or, more likely, save the result of Game->GetMIDI(), and feed it to Game->PlayMIDI() later)