If the music is on a music track and you have the "clip" button selected in the edit window you will not hear the music track, the clip will play and the scrubber will return to the beginning of the clip.

Select the "movie" button and you will be able to hear the music track.

Also is the music "live" as in recorded with the video, or is it music added from another source? Music added from another source in WAV format actually shows the waveform on the music track. I notice that last night when I processed a SmartSound (Magic Music) track in SonicFire Pro and exported it as a WAV file, then imported it into the media section of PD and dropped it on the music track.