Seems like the Enhanced Dialogue option when using HomePods as speakers is gone in the revised beta 3. I thought it was a great feature and worked well.
Edit/Update: After watching an episode or two on Prime Video tonight without the Enhance Dialogue setting appearing in settings, the volume increased on the next episode and sure enough the Enhance Dialogue option is back in Audio/Video settings.
Edit #2: I believe the change was due to my Apple TV auto updating to the Public Beta of TVOS 17. I was previously on the updated Developer Beta 3. Different build numbers as far as I can tell. TVOS 17 Dev Beta 3 is 21A5277j and TVOS Public Beta is currently listed as 21J5303h, which is what my Apple TV shows now.