When playing survival and creative, or idling in the menu screens, two of the music tracks will sometimes overlap. This could be any of the types of music, I have had at least 6 different tracks overlap another, and sometimes it'll overlap itself; start playing the same song again some time after the first has started.
Environment
Windows 7 - Java Version 7 Update 45 (build 1.7.0_45-b18)
Music tracks now loop. Its a new feature.