Hey there, I have a Sony A9G we bought about a year ago. In that time we've just been using the headphone jack, hooked into our stereo receiver. I've been trying to use the digital audio output just because I hear it's better quality. My problem with doing this is volume control. I cannot change the volume with the given Sony remote, the only way it's possible is to turn the big knob on the receiver (it's old and has no remote).
Any way it's possible in Android TV settings? I've poked around a bit and came up short.
The digital audio outputs unfortunately carry no volume information, so it won't be possible to control your old stereo's volume with your TVs remote, unless your old stereo has an IR receiver and you could then program the TV remote to control it. But you already stated the old stereo receiver didn't come with a remote, so that doesn't seem possible then.