This has been my life for about 3-5 months. I can't quite remember, I've been working on a lot lately. I recently bought a cheap 4k Samsung un55nu6900. I have a Sony 52kdlxbr4. I'm running on Xfinity with Cisco Gateway. Both TV are stock speaker setup essay writer
I decided to keep the Sony as it had fairly respectable sound levels. I have a Chromecast Gen 3 attached to it and usually play music via Spotify or videos via Youtube. Meanwhile I use the TV for news/youtube/yorno, etc. As it's got fairly weak speakers I tend to leave it at volume 0. And I play music often via Spotify on the Sony. I noticed that I could hear whatever was playing on the Samsung TV on the Sony while playing Spotify.
I went down the chromecast rabbit hole, the Sony rabbit hole the Spotify rabbit hole, even tried the xfinity rabbit hole, but the entrance was too small... I've then tried switching to playing music via Bluetooth speaker, (Braven805) while watching anything on my Samsung TV, Samsung s10+ phone or my cheap HP LAPTOP while muted. Always without failure audio from the muted devices will output as an overtone on my Spotify, Youtube, Samsung Music. Whatever I'm primarily listening to gets hijacked. It wouldn't really bother me too much except for the yorno. It's almost louder than the music.
I tried a Furman Power conditioner, all kinds of wifi settings and it even does it when devices are not connected to my network. I'm just out of ideas. Not sure if it's interference or something entirely different.