Have you managed to get this sorted? I just updated to TVOS17 and the problem is happening to me too. Like you, it seems to be intermittent. Sometimes I can get it working again by doing unplugging the ATV, but then it just stops again.
I did not that when I look at the sound output on my ATV, it shows TV speakers rather than the sounder, but when volume control returns, the ATV sound output shows Sounder.
I called Apple Support, but they have no idea what is going on, but made me do the usual stuff of trying different HDMI ports, different HDMI cables, etc but we still could not solve the problem.
Frustrating as everything was working fine until I updated to TVOS17