The DualDisc format was mostly used by the music industry in the last two years as a way to give their consumers an incentive to buy their products. It was, in short, a sales gimmick. The double-sided disc contained one side which was a CD and the other which was a DVD. The biggest problem with it was that the CD side didn't meet the specifications to carry the official "compact disc" label and wasn't even playable in many CD players! Out of five CD players I tried, the CD side of DualDiscs would not work in three of them. One of the players in which it wouldn't work was the CD player in my computer, meaning I could not import the songs to an iPod.
The DVD portion of the DualDisc usually contained the audio from the CD in multi-channel sound and about a half hour or less of related video (the artist in the studio, a music video, etc.). While it is nice for those with multi-channel stereos to be able to listen in 5.1 surround sound, those kinds of stereo systems are not in the majority of homes, at least, not yet. Most people buying a CD will listen in the car, a boom box, or on the computer, none of which accomodate the intricacies of 5.1 DVD sound. Moreover, niche multi-channel products like SA-CD (Super-Audio CD) and DVD-A (DVD-Audio) failed, in part because of the format war between them and in part because the market didn't exist for higher-ticket items that required higher-ticket systems (and the fragile market that did exist couldn't withstand a format war).
The music industry no doubt felt it needed to provide an incentive for people to buy their CDs, especially with sales falling. The music industry constantly points to file-sharing and digital downloads as the reason for falling sales. It is worth noting, however, that academic studies suggest that not only does file-sharing not hurt sales, it actually helps encourage sales. There is something dishonest about pointing to every file downloaded as a "lost sale," as if every person who ever enjoyed a song on the radio would have gone out and bought the record.
One thing is certain: Selling consumers CD/DVD hybrids that wouldn't actually play in a lot of CD players couldn't have helped sales. Memo to the music industry: Fewer disposable pop stars, more artistry; Fewer gimmicks, more substance.
More info: Wikipedia: DualDisc
