Same thing started happening to me precisely when 10.0.0 was installed. I am hearing this same sound on apps like AppleTV+, Plex, and others. In Plex, I can listen to music and watch trailers perfectly fine, but the actual movies fail. They are all AC3.
I watched a show last night on AppleTV+ and it was fine. My system upgraded at 7:08pm and tonight the same episode emits this strange sound instead.
I have found that if I manually override the HDMI audio setting to Stereo, the problem stops. I normally have Dolby Digital, DTS selected.
I have a Roku Ultra 4660X currently running 10.0.0 build 4177-46.