The volume will not work on my Apple TV remote suddenly. It began in a TV app and worked on other apps. Then the volume quit working completely the next day.

Thank you for those details.

Try restarting the remote as described here: If the volume buttons on your Apple TV remote stop working - Apple Support.

Verify that the path/​line-of-sight is clear for IR signals from the Siri Remote (1st generation) to the receptor on the TV.

Verify the battery charge level, and try charging if needed.

If you still need help after those steps, contact Apple Support. E.g. the remote itself might be partially damaged somehow. Then consider getting a replacement remote.

Do you get no audio at all, or is this about volume control (making changes to volume) with the remote?

Is this with the physical Siri remote, or through a software remote on mobile?

Verify that the audio isn’t muted.

What is your audio setup? Do you use the TV’s built-in speakers, or something else?

How are the components connected in your setup?

Verify the wiring (unplug and re-connect) for any wired parts between last working equipment and the speakers.

Apple TV 4K, 13

tvOS 13.x was 2019–2020 era. We’re on tvOS 16.3 now. Users are advised to keep their devices up-to-date, for best support, features, compatibility, and fixes.

Update your Apple TV - Apple Support

About Apple TV 4K and Apple TV HD software updates - Apple Support