According to this, Microsoft filed a patent in 2006 to "own" in-game soundtrack functionality. According to that report, the patent covers:
- receiving a request to launch a game in a gaming system;
- determining whether the game has a user-associated soundtrack;
- if the game has a user-associated soundtrack:
- launching the game; and
- playing the user-associated soundtrack;
- if the game does not have a user-associated soundtrack:
- allowing a user of the gaming system to select a soundtrack to play while the game is executed; and
- playing a default soundtrack if the user of the gaming system does not select a soundtrack.
That article concludes that Sony hasn't been allowing custom soundtracks because of the above. Apart from in a few games. Which are magically exempt from the patent. Remember all that stuff about Nintendo owning a patent on rumble technology? No? That's because that didn't make any difference either.