Beware of ads that use inaudible sound to link your phone, TV, tablet, and PC
arstechnica.com2015 article. Something more relevant might be facebook getting a patent to control your phone via tv audio.
https://metro.co.uk/2018/06/22/facebook-wants-hide-inaudible...
What counter-measures exist? (e.g. an audio detector listening above 20 kHz?)
Permissions is the simple, albeit flawed, answer.
It doesn't matter if your TV makes a noise if nothing is listening for it.
Deny access to your microphone. There are very few good reasons for an app to access it.
Use your firewall. Your data belongs to you, there are few good reasons that an app should be connecting out to services like SilverPush.
Unfortunately, this usually requires domain knowledge ("who is trustworthy?"), which is why the answer is flawed.
Would using something like pi hole at home help in that regard?
Yes and no.
Your pi-hole can block things, but if you leave home and the app framework has cached communications, what happens then? Can it now push the data?
Canvasblocker on Firefox.
https://github.com/kkapsner/CanvasBlocker/releases
Its a never ending against marketing parasites.