Apple's online iTunes music store is now the number-two music retailer in the U.S. behind Wal-Mart Stores as measured by unit volume, market researcher NPD Group said Tuesday. NPD said that iTunes moved into second place due to the amount of music it sold during 2007, which was based on a 12-track CD equivalency for song downloads.

iTunes Rises to No. 2 Music Retailer Spot as As CDs Drift Away