I have an older Sony DVD Home Theater System and a brand new Roku S-Series 4K HDR TV. I have my COX cable box going into the AV system then the ARC output from the AV system going to the ARC input on the TV. Everything works great, except if I am streaming from the TV through the AV system I hear audio from the cable TV input instead of the streaming audio.
The ARC is working fine between the TV and HTS while watching cable TV with audio through the HTS. The cable box is connected to HDMI 1 on the HTS and the TV is taking the video output from the HTS and, the way I understand it is, the audio is returned to the HTS from the TV via the ARC channel.
The HTS has an HDMI ARC output - the TV has an HDMI ARC input. The setup menu on the HTS only allows for selecting HDMI 1 or HDMI 2 as the input. I can find no way to select the HDMI ARC output port on the HTS as the input source when running a streaming app on the Roku. I can make it work if I put the HTS function on TV and use the optical sound out from the Roku to the HTS but haven't been able to make it work just using the ARC.