The answer is I suspect surprisingly simple; Is the new track you added "City Night"? If so, it seems now to be just extremely quiet background sounds of the countryside (birds tweeting and crickets are about the only thing you can make out), and then it stops.
Selecting the SAME song as is currently playing will also refuse to re-start it again, which can also make the box seem like it's broken with either the above track, other tracks with a quiet introduction, or tracks with an infinite empty space ("Conversation with Gwenno" is one) after the music ends... because it's hard to tell the track is actually playing at that point still.
Changing track too fast after selecting a previous one can also make the handle spin but the new track won't start. Give it a few seconds.
And one final bug, just in case the devs look here; "Northern Forest" from the new songs is STILL pointing to "Battle On Stones" instead. Give us the true "Northern Forest" track