Bingo!
It's an LG UP870. I checked the settings for the audio. It had Auto, PCM and DTS-Re-Encode. The unit was set for Auto. So, I moved it to PCM, then played the Sonic 2 disc. It worked!
Then, I moved it to DTS-Re-Encode, played the disc, and it also worked!
For some reason, having it set to Auto didn't allow the audio to come through.
I tried it with the Logan disc. Both 1 ENG and 1 ENG audio channels worked, as well.
You have solved the mystery! Thanks for being persistent with this. Plus, between you and Rich86, I learned a bunch of stuff.
Thanks again!