(UPDATED) At a Los Angeles event scheduled for this afternoon, Google Music is expected to enhance its service with new mobile and sharing features plus a download store including the catalogs of Universal, EMI, Sony and indie aggregator Merlin. Warner Music Group remains the one holdout; and Sony just came aboard last night, according to multiple sources.  Here's what to look for during today's Google Music announcement:

• Finally, a Google download store - sans WMG.
• Some sort of music sharing function.  Google execs have suggested that music sharing will be a central theme, exactly what that means is anyone's guess.
• Mobile - We know that T-Mobile is throwing the official after-party in the launch venue. We also know that mobile is at the center of Google's Android platform.
• Expect more robust integration with their Google cloud music locker.