a 5.1 mix is music mixed using 6 channels instead of 2 (stereo). Front left, center, front right, rear left and rear right, and a channel just for lower frequencies. Much like the way dvd movies are mixed. There is a growing? trend to release dvd versions of records, using this mixing technique. If you have ever seen a Dual Disk, thats what it is, a 5.1 mix on one side, or a stereo, 2.0 mix on the other. Sometimes, bescause these are mostly listened to in a home theatre setting, they will have visuals for the t.v., or just song listings.
It can really bring a lot of personality to a music recording, by being able to flesh the sound out around you, and is also great for live recordings. (sometimes putting mics in the back of the room, and mixing those to the rear channels) Gives you a 3-D? sound
These mixes can usually be found on live DVDs', or on DVDA (dvd audio, which requires a DVDA player), or on a regular dvd, that plays on any dvd player (usually giving you the option of playing a 2.0 mix, or a 5.1 mix) using dolby, or dts surround.