This is something I've noticed on my Roku TV. When I begin playing a video on any service be it YouTube or Disney plus or whatever there's about a 5 second delay between when the video starts and when I start hearing audio. Now I stress here that the audio isn't out of sync, it's playing the right thing for that point in the video, there's just silence at the start of anything including ad breaks. I'm using the hdmi eArc to connect my sound bar to my TV.
The weirdest thing is that after the most recent update the button sounds now work when I'm navigating the menus, it used to also take a few seconds of pressing buttons before I'd hear anything, but now that works fine.
I've tried playing around with the settings but that hasn't fixed the issue.
Roku does not make TVs. I have a TCL Roku TV with no such issues. First thing to do is to take your soundbar out of the picture. Disconnect it and only use the TV speakers. If the problem does not occur then the issue is with your soundbar.