Apple has announced that iTunes is now the second-largest music retailer in the US, trailing only Wal-Mart, according to the latest data from the NPD Group. The NPD Group’s MusicWatch survey captures consumer reported past week unit purchases and counts one CD as representing 12 tracks. Apple also announced that there are now over 50 million iTunes Store customers. “We’d like to thank the over 50 million music lovers who have helped the iTunes Store reach this incredible milestone,” said Eddy Cue, Apple’s vice president of iTunes. “We continue to add great new features like iTunes Movie Rentals to give our customers even more reason to love iTunes.” iTunes has sold more than four billion songs, and offers the world’s largest music catalog of over six million songs.