Open Mic plays tracks from a random selection of tracks that have not posted yet, for most tracks listened you get one heart.
The amount of hearts you can get from Open Mic is -- like your daily amount of hearts -- your level + 9, with one important understanding, which is that you can never have more hearts than level + 9 by listening to Open Mic, once you get that amount of hearts, Open Mic will start giving reputation points instead.
To clarify: If you are a level 10 you can get 19 hearts from Open Mic. If you start listening to Open Mic while you still have 8 hearts left, then you can get only 11 more hearts from Open Mic, unless you use up more hearts.
The tracks Open Mic plays depend also on what level the adventure meter is set to. In general, the higher the adventure level, the less popular a track is, or the less activity it has seen. While Open Mic claims the heart reward grows with higher adventure levels, in practice it does not seem to matter what level Open Mic is set at: you'll get 1 heart per most tracks played.
The Rack Edit
The Rack is the predecessor to Open Mic, it works mostly the same as Open Mic, but lacks the adventure levels and comes with a link to block a track from showing up on your Rack again. Earning hearts from the Rack only seems to work if you selected any genre but "all" from one of Popular pages.