: Just curious, but are the soundtracks composed by Bungie for Halo now the
: sole property of Microsoft with the latter's acquisition of the
: franchise's rights?

Music copyrights are fiercely guarded by artists and music companies. They are usually just licensed to games companies for use under very limited terms. For MS, being a computer software company, it wouldn't make sense for them to try and buy out the rights because they have no experience in it. They'd have to outsource that operation to an established company anyway to make it economical. That's if O'Donnel/Salvatori were even willing to sell, which I doubt because it's their bread and butter and they'll want the copyright for the rest of the (many-decade) term.

Actually, the game content usage rules specifically mention:

But we’ll confirm right here that the music from Halo 3 is available for your use in non-profit ventures thanks to an arrangement with O’Donnell/Salvatori, Inc., composers of this iconic theme.

MS wouldn't need to do that if they owned it unless the terms of sale had some consultation clause. It'd be simpler all round for O’Donnell/Salvatori to keep the copyright and just license it out.