iPod will not play

My iPod 5th gen enhanced won’t play content anymore.

All the buttons and click wheel are functioning, the iPod syncs with iTunes on my PC and I can play the content with iTunes. When I press the play button, the play icon illuminates and pressing again shows the pause icon but the elapsed time/time remaining counters and progress bar don’t move/change. All buttons and click wheel pass their tests in iPod Diagnostics. The hard disk has been removed and tested. It is error free. I have also wiped it and restored the iPod with iTunes all to no avail.

I installed Rockbox to see if play would work with that but unfortunately I get the same no play result.

I was wondering whether this could be a firmware fault and if there’s a way of installing an earlier firmware version to see if it clears the fault?

All suggestions gratefully accepted to try and revive my once trusty iPod.