Main site show top ratings for items in sub-sites

I set up a Network install of WP and am using ProSites to let users create e-commerce sub-sites with MarketPress. The most common product will be .mp3 downloads. My new challenge is that I want to let visitors rate the .mp3's on the main site. I'm concerned that the ratings will be sub-site specific instead of network specific. In other words if I rate a song from Band-A I don't want it to be compared to the rest of Band-A's songs. I want it to be compared to all of the band's songs.

Thanks.
Jeremy