[Ambientnoise]

I did exactly this in the first lockdown. The usb idea is good but my TV powered the usb outlets all the time. I did look at detecting audio via the headphone connector and toslink too.
In the end I went for a current transformer on the mains feed followed by rectifier, smoothing, threshold detectors and some careful timing before a relay, switching power to the audio system. The threshold/timing has to take care of several aspects such as normal running with light and dark pictures, black screen whilst updating, loading Netflix, loading schedules etc. I ran the various scenarios and noted the output and time delays from the rectifier. This is in a small box, built around op amps and cmos 555s, continually powered and works well. I did dabble with an arduino based unit for fun but gave up on it.
The small current transformer just has a load resistor and a vdr across it before the rest of the circuits.
You can get small current transformers, on a mini pcb intended for Arduino etc, in the usual places.

Ken