Rhythmbox is one of those apps in need of an icon yet nobody can seem to do much with. Here's my attempt.
The music note and flat notation are intended to look like rb for Rhythmbox.
I've added a second screenshot to have the community help me finalize the icon. Once I get some feedback I'll do the icon in the various sizes and write a script to plug this into your icon theme.
Simply vote for your favorite version from screenshot 2 and let me know if you like 1, 2 or 3 best.