Richard is getting a strange buzzing noise as he watches videos from his Roku 4K Ultra. It happens when watching YouTube. He'll turn down the volume on his home theater system and turns up the video on his Roku remote. It's tinny, but it's clear with no buzz. Leo suspects that there's a mismatch of codecs. There are so many different video codecs now that this can happen. So you could be watching a movie with DolbyVision HDR, and the encoding is confusing the AV Receiver. It could also be an analog problem. Check your speaker cables. Also look to be sure wires are crossing and touching. Double-check to see if the speaker cables are also properly seated and connected.
Leo also thinks the volume control of the ROKU is turning up your TV speakers, leading Leo to believe it's the AV receiver settings. Check the settings to see what audio it's putting out.