But imagine Ultima Online with a dynamic score that constantly changed, one that was downloaded on the fly. You can do that with MIDI, but with the various cards offering slightly different instrument sounds, it would be nearly impossible to guarantee the music would sound identical on all systems. They could download samples to the cards, but again you're looking at a larger download.
Another possibility is coming up with streaming MP3 audio, effectively turning Origin into an online radio station. Certain events, such as combat, might trigger these custom tunes, and they could be swapped out every few weeks.
Ultimately, online gaming is all about minimizing bandwidth, which tends to collide with all of the THX standards. How all of this may work in the future is the job of Matt Mitchell, who supervises the creation and recording of all audio for Origin. He'll get started on this task once Ascension is out the door.