Edit: to be clear, this post is in reply to this thread: https://community.roku.com/t5/Solving-playback-issues/Audio-doesn-t-play-at-the-beginning-of-videos/... but somehow it ended up being its own thread.
I had a very similar issue and in my case it turned out to be the screen reader feature! I guess my toddler turned it on at some point. "But," I hear you say "I don't hear a screen reader." Well neither did I! It turns out that if you enable audio leveling as well it somehow disables the voice of the screen reader. Why? A bug, probably. However, the effect that was still there is that it lowered the media volume (by a lot) whenever the screen reader wanted to say something (basically whenever anything UI-related happens, like starting a video, opening a menu, or even when it displays the "skip intro" button).
When I turned off audio leveling it started narrating the menu to me and I realised what was up. You can turn the screen reader off by going to Settings -> Accessibility -> Screen reader. Hope this helps.
