
They appear to be going up in alphabetical song name order, so today we've got:
Jimmy Bravo - All Right Nippon
Jackie'O - Alleluia Japan
Dominique Pratt - Always In My Mind
Priscilla - America
Betty Blue - Angel
Maria Short - Another Devil
Regina - Baby Blue
Maria Short - Bad Angel
More to come, I'm sure! I haven't seen any sign of Eurobeat Masters songs going up just yet, but maybe they're in the pipeline for after Vibration?
Slightly more expensive than iTunes again, but nonetheless there's instrumentals/vocal-only versions for those who like them!
If they carry on in alphabetical order, we can expect the next batch to include (most likely): Bad Love, Bambino, Bang Bang, Be My Lover, Be My Tears, Beautiful Day, Big Big Heart, Big Mig, Big Time... OK so you could go rather a long way with this list!

They seem to have skipped Back To The Future with this list, but it's probably temporary.