Panasonic Plasma - Controlling volume through optical output

I have a bizarre issue with a new Panasonic Plasma TC-P5032C. (From my searching, this appears to be a 2011 Panasonic X3 model sold only through Target). I have the optical output going to a SONY HT-CT150 sound bar.

My toddler messed around with the remote and when I took it back from her, the Volume buttons controlled the volume going through the optical output to the sound bar. Whenever I clicked, it would say something like "Home Theater Sound Volume", and would show the volume bar going up and down. Great feature!

Fast forward 2 hours, and I have moved my whole setup to a different room. The volume buttons now controlled the internal speaker volume. I assumed this reset when I unplugged the TV. I went into Audio Settings -> Advanced Audio. I changed the "TV Speakers" setting to off. However, when I pressed the volume up/down buttons, I got no change in volume. Only a message on the screen saying "Speakers Off". Where did the "Home Theater Sound Volume" message go?

What is going on? Looking at the manual, it seems that only Panasonic Viera compatible devices can be controlled with the TV remote. How was I controlling the volume going out to the sound bar two hours earlier? This one's got me stumped.

Thank you!

Things are starting to make sense. Currently I have 2 separate HDMI inputs going direct to the TV. Before I may have been using the 150 for HDMI switching. Very interesting. I've never heard of ARC until now, but that sounds promising and I'll see if that is/was what happened. I would like to be able to control the volume with the TV remote.

Thanks!