Just in case this helps somebody:
I'm brand spanking new to Construct and am currently messing around with sounds and music using the stock assets that I downloaded with the paid version. The sound effects were showing in the play list in the Play > Select Song action, but music wasn't. Turns out that I had imported WAV files for the sound effects, which are automatically converted to both m4a and ogg, but had only imported the m4a music files. Construct wants both formats apparently for browser support. Once I imported the corresponding ogg formatted music file everything was fine.