Retail superstore Wal-Mart yesterday opened its online music store, Music Downloads. Like all non-Apple stores, Wal-Mart uses Microsoft's Windows Media Audio (WMA) 9 format to encode its music, but the company is offering tracks at a somewhat discounted rate, at 88 cents apiece. (Albums typically cost $9.44.) Apparently, the current version of the store is a test, and Wal-Mart is hoping to receive some customer feedback before the music store's wide-scale rollout in 2004