When using analog connections, your Xbox is outputting a stereo signal; however, through matrix encoding schemes like Dolby Pro Logic, you can get some surround and center separation. Your surround receiver then decodes the matrix and plays back sound from each speaker. This allows the Xbox to play sounds that come mostly through the surround speakers or mostly through the center channel, but as there are only two discrete channels, you couldn't have a sound that is, say, only in the center but not in the left or right channel.
Now, what this means is that when you are playing music (the vast majority of which is recorded with two channels), the surround decoder puts the music mainly into the left and right channels, but some will "leak" into the center and surround channels because the center channel essentially plays anything that is in both the left and the right channel, and the surround channel(s) play things that are in the left and right channels, but out of phase with each other. If that doesn't make sense, it's fine- just understand that when sending surround sound over an analog stereo connection sound will "leak" into adjacent channels.
Now, when you switch to digital sound, each channel is discrete. Sounds can come from any channel individually with no leakage into the other channels. This allows much better and more precise surround effects, and, for example, dialog in movies will come only from the center channel, not also from left and right, which improves clarity. However, when you play back a stereo recording, like most music, only two channels are used.
If you want the music to be played back through all of the channels, try changing the mode on your surround receiver to something like "dolby pro logic". There may also be a mode specifically designed for playing stereo music on all of the speakers; check the receiver's manual.
It is also possible that the xbox has such a feature on its end (which would decide how to play back the music on 5.1 channels and then output each channel to the receiver), I don't know because I don't have an Xbox.
Hope this helps!