Same problem, posting here for another data point and to follow.
On a few occasions now I've loaded ~10 songs, all single tracks, and they work perfectly. After powering down and upon reboot, they look as though they are playing but there is no audio. And the Matrix screen song meter does not show activity. This seems random, sometimes they do work following reboots.
I have also done a number of various sequences of backing up, factory resetting, restoring from backup, reloading songs, etc. I've yet to find any repeatable pattern.