I have discovered that content recorded with PlayOn Home, then imported to Channels, then played on the Apple TV Channels client is completely missing right channel of audio. I thought this would be a breeze to troubleshoot by seeing whether it is the source content or the client that makes the difference in audio.
But results below are puzzling. It seems that both audio channels are in the .mp4 file as recorded by PlayOn. But Channels via Apple TV has no problem with any other content. And the Apple TV has no audio issues with any streaming service.
To be sure I had L/R audio information, I used PlayOn Home to record a YouTube stereo test video. Playing this video showed all OK on all the same OK cases below, but interestingly enough, the right audio information came out of the center speaker while the left came out of left front. The speaker assignments on my Denon AVRX-3400H are all correct as verified through the built-in speaker test.
The detected audio for all content I play through Channels via Apple TV is Dolby Digital in all cases, and only the PlayOn Home content behaves this way. I'm at a loss where to look next.