Walmart.com is launching its own digital music download service. When the world's largest retailer gets involved, you know music download services are a good business prospect. Walmart.com will be in a partnership with a company that recently bought Liquid Audio's digital music business. The new service will contain 200,000 tracks to start, and could launch as early as next week.According to a report by Media Post, "Jupiter Research estimates $35 million in legally downloaded music will be sold this year. That's small right now compared to the $12 billion music industry, but digital music is climbing the charts with a bullet."