I'm not sure if this is a receiver or TV problem but since it's an audio issue, I'll post it here. If it should go somewhere else, please let me know.
I just got a TCL 50S425 with built in Roku and I have it connected to an Onkyo HT-R390 receiver via the HDMI port with ARC. Everything works properly except that when I adjust the volume when watching a Roku channel, it mutes for about 1 second per level. This is quite annoying.
When I watch the satellite TV "channel" from the Roku menu I don't have this problem. The satellite receiver is connected to the Cable input on the Onkyo which has a single output to the TV. The Onkyo requires the TV/CD input be selected to hear the ARC audio. Interestingly, the Onkyo changed to that input automatically when I set up the TV.