5.1 Music Discs, also known as DTS Audio CDs, are a surround sound music format.
Physically, a 5.1 Music Disc is an ordinary compact disc (unlike e.g. SACD or DVD Audio discs, which are DVD type discs). It is formatted into tracks like an ordinary music CD. A normal CD or DVD player will recognise it as if it is an ordinary audio CD. However, the data on the disc is DTS encoded surround sound, which the player knows nothing of, and so when the player attempts to convert it into analogue audio, it will merely produce an unpleasant scratching noise.
In order to translate the DTS encoded data into sound, the player needs to be connected via its digital-out port to a DTS decoder. Most modern home cinema equipment can handle a DTS signal, as increasing numbers of DVDs offer DTS soundtracks.
Alternatively, a properly equipped and configured PC can play the CD using a software DVD player.
The output is five channels (plus a subwoofer) of glorious DTS surround sound.
There's only a relative handful of albums available in this format. Mostly, they seem to be re-releases of older material, remixed into the surround format. I have listened to the DTS version of Steely Dan's Gaucho and, in my admittedly biased pro-Dan opinion, it sounds absolutely gorgeous. Half the band, and, appropriately, the backing singers are back here, and Donald and the other half are over there. Very occasionally some percussion or other 'effect'-type sound shimmers around the place a bit but mostly the mix is gimmick-free. Babylon Sisters has never sounded so good.
However, reviews of some other albums suggest that not all material is equally suited to a surround remix. Steely Dan's highly polished studio sound benefits quite naturally from the extra channels, whereas the rough and ready sound of, for example, Sting's Roxanne sounds awkward stretched across so many speakers.
DTS Entertainment claim that production of 5.1 Music Discs will continue in parallel with DVD-Audio, which also supports the DTS format. 5.1 Music Discs have the advantage of being just as cheap as ordinary CDs to manufacture, but DVD-Audio and SACD both offer advantages of their own. It remains to be seen what, if anything, will eventually displace the humble CD as the most popular music format.
See http://www.dtsentertainment.com for the catalogue of available albums. Bodacious cowboys, such as your friend, will never be welcome here.