You have an older TV, so there is a good chance your TV cannot pass through Dolby Digital 5.1 audio from an external media device.  What are your TV’s audio settings set to?

The options are Auto or PCM it is set to Auto

There is a community user here who says that TV will pass-through DD5.1. See here:

So it should Play if the connected peripheral/App is set to output that audio codec.

Reading through the thread some more it looks like it only works if the Arc is connected to the TV using the HDMI/Optical Port converter. So perhaps give that a try.

Check your Xbox and Chromecast settings too.

On your TV, be sure Simplink (CEC) is turned on and Sound Out is set to Audio Out (Optical/HDMI ARC).

Is voice control the only functionality I lose with Optical?

There’s no voice control of the TV and no Atmos audio, otherwise you should gain your Disney+ if the 5.1 does work for you.

You lose the automatic volume control via HDMI-CEC when you use an optical cable. The Arc can be controlled via your remote too, though this might require some extra set up steps….