We've had lots of chatter on the IndieBoard about songwriting and mechanical royalties, and while I was knee deep in the whole formation of SoundExchange because I worked in radio at the time, it's amazing that word of SoundExchange is still not out among many working musicians.
Here's the deal:
You already get a little royalty from your PRO if your song is played on the radio, whether it's over the air or on the web. But you get an extra royalty for internet or satellite airplay, which is tracked by an organization spun off from the RIAA, called SoundExchange. This group does not have anything to do with downloads or iTunes sales, only the royalty mandated by the Library of Congress for streaming audio.
So if you're not already in their database, and you've got some tunes hitting college or non-commercial radio, get yourself listed and get a chunk of the revenue for yourself.