How can I have a music track fade to a low gain/volume while I talk over it with a second audio track in Adobe Premiere Pro CC?
Ducking is a great solution for broadcast audio (for example, muting crowd noise when a commentator is trying to explain what's happening on the field).  In production, the limitations of ducking become apparent: sometimes it's too much or too little, sometimes it needs to be faster than the default, or slower.  These are things that are not necessarily obvious the first time you listen, but become obvious after the second or third listening.

Broadcast audio production presumes you hear things only once, so the limitations of ducking are acceptable.  If you are producing material that people might watch more than once, you really should do the ducking manually, setting audio keyframes and moving the volume manually to maximize not only quality and intelligibility, but also emotion and impact.