My suggestion is to put the in game music up on the website (either only for people who bought the full version, or to everyone). Then, depending on your time and people's interest in it, maybe make and sell an album that has higher quality, remixed, and expanded versions of the tracks. That way people will be able to listen now without starting up Aquaria and leaving it in an active window, and you'll have a project that can maybe bring in a few extra dollars.
I don't know how that sounds from your end of things, but I think that would be a way do both the free download option and the soundtrack for sale option.