Is it possible to change which output the audio is played through? Is it possible to reroute audio (i.e. music) through the phone earpiece instead of the speaker? I've searched this site and Google extensively and haven't found anything yet.

If it matters, I'm on the HTC Evo 4G LTE (Sprint). I'm also rooted and have CM10.1 on my phone.
You can simply use an app like Earpiece. It does what you want to do. You can pop Earpiece up on top of another application simply by holding down the SEARCH key. then check Audio through earpiece



On a side note, USE AT YOUR OWN RISK. Playing audio at high volumes, especially for a prolonged amount of time, can damage speakers and/or hearing.