MTV is expanding its digital presence. The network is expected to launch a $50 million push to sell downloadable songs in an open source MP3-friendly format through its Rhapsody digital-music service, a joint venture with Seattle-based Real Networks. (Rhapsody also serves as the outsourced digital-music retail provider for Yahoo.) The move, which includes deals with all four major record labels, is designed to give Apple’s iTunes a run for its money.