I have a TCL tv and fixed mine by, while on YouTube I pressed the star button on the Roku remote, scroll down to Volume mode and changed the setting to off. After that YouTube sound was back in.
A few people in here figured out that when you pause youtube and press the * the sound comes back for that video. You need to keep doing that unless you do what RED808 said to do... while on yt go to volume and turn it off, then the sound is on.
It's still a little wonky inbetween sometimes and I hope they fix it but for now this works the best.
THANK YOU EVERYONE who figured this out.
I posted the solution many many messages ago: Press *, scroll to volume mode and switch it to off or normal while a video is playing and the sound should return instantly. Make sure it's not up loud so speakers don't blow. Unfollowing this now because I'm getting annoyed that people are not reading and doing as I suggested, besides a few.
I press the star button on the Roku remote scroll to “volume mode” and change the setting to off. If it is already set to off try changing it to another setting and back to off.