The world`s largest retailer, Bentonville-based Wal-Mart Stores, plans to launch its own online music downloading service. Wal-Mart will look to its core customer base with its new online music service, offering a library of largely country songs for less than the standard 99-cents per song. The retailer already sells 20 percent of the nation`s recorded music, and rents D-V-Ds online. Analysts say offering music opens the market to a new demographic. Wal-Mart`s service may be online as soon as next week and carry about 200-thousand titles -- half the library of some other downloading sites.