I had this same issue. TCL 4 series tv. I have dish and I went though the Dish remote into audio settings and set the Dolby digits to Yes after reading this just to try it and it worked for me. (I couldn't find the same option with the roku control).
Sound delay on your TCL U50S6906
Some TVs like your TCL U50S6906 have a “Sound delay” option.
If this has been set on your TCL U50S6906, go to the “Menu” of your decoder, and go to “Settings”.
Then select “Picture & Sound”. Continue in “Sound Adjustment”, and deactivate Dolby Digital on the HDMI output by selecting “No”.
I believe the issue has to do with the active content review (ACR - or whatever the acronym is) in which the Roku technology attempts to recognize what you're watching from whatever source you're using (cable, game console, PC, etc) to provide suggestions for other shows. That would explain why the TV makes a noise when changing stations and not changing input sources such as cable to gaming console. I'm not sure how this technology goes out the door as it should have been caught relatively early by Roku while testing.
I have a friend who got a 50S425 TCL Roku TV last month and I noticed this issue from the first time since I helped setup their Dish from their old non-smart TV to this Roku TV. I called TCL for them and they informed me that Roku has a test software for this and needed the owner's approval. After doing the update, that seemed to resolve the issue.
Try calling the manufacturer of your Roku TV or maybe Roku directly to see if yours is software related.
I just bought a TCL Series 4 50S425 from Amazon and connected it to Spectrum Cable for my TV service through an HDMI port. This TV beeps everytime I change the channel and it also continuously beeps when rewinding. It's very annoying. My Spectrum menu does not give me the Dolby prompt discussed above. So I can't try that. I spent 2 hours yesterday in an online chat session with TCL customer support trying to eliminate the beeping with no success. They finally said it was a Spectrum issue. I highly doubt this because my Vizio and Samsung TV's work fine (no beeping) with Spectrum. HELP!
The easiest fix to solve the problem is to download the spectrum app and use it instead of the box. I took my box back and just use the app now and no more beeping but the app has some drawbacks I'll have to get use to.
I'm having the same issue with two TCL 50S525 TVs and Cisco 4742HDC and 8742HDC cable boxes from Spectrum (formerly Time Warner Cable). It also happens whenever I fast forward or rewind on DVR, then hit play again. Are there different cable boxes that might help? Or is that "test" software from TCL still out there?