i have also tried optical and Bluetooth as audio options. There is always a predictable 1sec audio outage every 3-6 seconds on Orcs Must Dies 3 for Stadia with this setup. As soon as I switch to just internal tv speakers the audio issues are fixed.
I think that would be also great if you leave this as feedback in the Stadia App or on stadia.google.com as you pass it directly to the Stadia developers.
If that Audio lag is only happening in Orcs must die 3, I would also recommend to contact the developers of that game, so that they are aware of it and can work on a fix. Because if only this game is affected, it might be a bug coming from the game itself.
Sorry this is my first time reporting a bug for this platform. where else should i post this? I don't see an obvious bug submission form for the platform or a game specifically. I have played 4 other games now with no sound lag. It really does appear to be a bug with OMD3