Apple own the stereo download space because they are the champions of creating superior user interfaces, in this case in the form of the iPod. When iPod became uber-popular, iTunes ruled the industry.
For surround music there is no iPod, no other user interface than your browser, a library S/W and your surround music system. That is why I believe there is a play here for anyone willing to make the investment. Create the download market space, start the revenue, relief the music labels of making elaborate surround music albums and grow the audience. Hit Play!
In addition to a user-friendly interface, the "niche" that apple has filled is one of a "convenience listener": people who want to quickly download music to put on their portable player (as you say, the iPod started this service) or play back on their small iPod-boom-box or PC speakers for friends.
A 5.1 listerner isn't jogging. He/She isn't going to get 5.1 out of a boom-box. The 5.1 listerner is sitting in a chair in the middle of a 5.1 speaker array, typically in a high-end audio system. This consumer isn't convenience/portable oriented; he/she is quality oriented and is most likely a "collector" rather than a mere "consumer" of music. I doubt that there's a strong need/market for downloading among such a group.