I’ve experienced this on a newer TCL Roku TV - but it happens with DLNA as well, not just Plex, so I think it’s something with the Roku (on both 10/10.5).
Do you have an external speaker setup or is this happening with the built in TV speakers?
My silence at the start doesn’t show up if it’s TV speakers and is a lot less noticeable or non-existent if I have the audio output set to Passthrough instead of Auto - and that has solved other odd audio issues I’ve had with other Roku setups.
I have a different audio issue with Passthrough I’m trying to get sorted out (not a Plex bug) but it’s less of an issue overall. Reported it here in case it’s something Plex folks can overcome. 
You can also try fiddling with the Direct Play settings - sometimes turning that off (which barely impacts playback - sometimes some subtitle situations) can resolve odd audio issues.
