Haha, to be perfectly honest, I didn't even know you could sell soundtracks on Steam!
As for the HQ tracks in the actual game, they're exported at such a high rate that putting them in the game would make download time for new users a nightmare. We ran into problems with this with our puzzle game "Tidalis". People were complaining that the download time was too long, and it was all due to the file size of the tracks.
But, that being said, I try my best to make the quality of the in-game music as best as it can be without having gigantic file sizes!