See If HDMI ARC or eARC isn't working as expected Use HDMI ARC or eARC with your Apple TV 4K
When you set this up for the first time you need to make sure you enabled HomeKit & AirPlay in settings and assign a room that’s the same room as HomePods and all devices are signed in with the same Apple ID and connected to the same home wifi network.
- If using Ethernet on this Apple tv, test connecting it to wifi.
Check Play audio on HomePod using an iOS or iPadOS device and set Allow Speaker access to Everyone
I’m wondering if somehow you’ve missed some of the settings listed above.
Also make sure all software is updated to the latest Apple TV, HomePods and the device used to set these up, if this is an iPhone.
You might have to turn on ARC, eARC, or HDMI-CEC in the settings of your TV
https://care.onscreeninc.com/en/articles/5750759-how-to-enable-hdmi-cec-on-various-tvs#