First I'll give a quick rundown on my setup. HTPC downstairs running Plex Media Server. Wired Ethernet throughout the house to different rooms. Issue is in my main living room with my Roku Premier. I have my Roku HDMI out into my Harmon Kardon Receiver. From there it goes HDMI out into a hi end Samsung Sound Bar HDMI in. Then HDMI out again from the Sound Bar into the TV HDMI in. My Bell Sat Receiver also outputs into the Harmon Kardon Receiver and I have never had an issue. I just got my Roku and I run Plex Media Player on it. Everything runs beautifully with no issues except for one. I can only get audio out of my sound bar when the Roku is set to Stereo. If I set it to HDMI, or Auto, or any of those I get no sound. The weird thing is is that when I play a file with say, DTS Master Audio, or Dolby Digital, my Harmon Kardon display changed to show its receiving that signal, but nothing out of the Samsung Sound Bar. Obviously I would try and trouble shoot all the other devices but the thing is I used to use an old WDTV Live player with no issues outputting the proper audio. It also works perfectly with my laptop hooked up playing through the home network. Even so I know it's not Plex, I have a Rasberry Pi 3 running Rasplex and it works perfectly with all audio formats. Just wondering if anyone out there had a similar issue and knows of some setting I'm missing. If I can't sort this out I'd rather scrap my Roku and just build another Rasberry Pi 3 instead of pulling HDMI's out of the wall and replacing them one by one. Just odd that everything passes the audio straight through but not with the Roku.
That's in advance if you've gotten this far, that's a long read.
Silence from a playback device frequently means that the playback device (your soundbar) cannot decode the audio stream, sometimes also evidenced by receiving audio when the source device (Premier) is set to output PCM stereo only. Do you get the missing sound if the Premier is plugged directly into your soundbar (for troubleshooting purposes)? Most if not all Roku devices have no decoding capabilities, but I seem to recall that the WDTV player line mostly had those capabilities, which may explain the behavior discrepancies you observed .
If you are using a quality soundbar, why are you using the H/K in the stream? Just for HDMI switching? It sounds to me that either the H/K isn't passing the audio through, or your soundbar doesn't support Dolby Digital Plus. Perhaps a combination of the two. Most apps/channels now use DD+ for their audio, so your audio processor has to handle it. When you change to stereo, you remove the DD+ stream from the equation.
Dan Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.