Unplugged what, and for how long? 
 

 The TV is generating the audio, or at least passing it through to the Arc. If you haven’t rebooted all the devices, you’re likely not to have gotten the fix on the one causing the issue. 

I've unplugged everything. TV, other HDMI devices and the Arc. At least for a few minutes. I then connected all as it was and did Trueplay. 

And when you say reboot do you mean just restart or factory reset? I've restarted the TV, checked settings etc.

He means restart, not factory reset.  Factory reset should only be done after getting a recommendation from Sonos directly.

Does the Arc play streaming audio (not from TV) correctly?  If you have another TV in the house, have you tried the Arc to that TV to see if the issue still occurs?

I would go ahead and replicate the issue, submit a diagnostic, then contact Sonos directly.  The diagnostic will give them good information about what’s going on.  It could be damaged speaker, could be damaged TV, etc.

I’ve already done that. Initially they told me to check to unplug, Trueplay etc. Then they told me to check the correct settings for the TV. Now they are telling me to reset the Arc. I will do that.

From my experience diagnostics doesn’t tell them anything. A couple of times they just told me they can see the signal the Arc is receiving not much more.

It’s the luck of the draw with the support person. The diagnostics are quite good. If the diagnostics are clean, it’s not likely that this is a SONOS or network issue.

In fairness, the diagnostics may not be able to indicate that an internal speaker has been damaged and does not sound good.