The cable that connects to av input is breakout cable like this:
3.5mm AV Male to 3RCA Female M/F Audio Video Stereo Jack Adapter Cord Cable | eBay
Im dealing with the same situation, HDMI 1 on the Roku TV is listed as "ARC HDMI", and the Bose 3-2-1 series 2, has "HDMI" Port, not marked ARC (Irrelevant, but in the event someone may not know the "ARC HDMI" serves as regular HDMI port when used with device that is older and has only regular HDMI port). I used an HDMI cable from HDMI port 3 to the Bose HDMI port and DVDs play on the tv, and sound comes through the Bose perfectly. However, im unable to get regular TV programming sound to come through the Bose 3-2-1 system. I plan to get the optical link connector today and connect the Tv and Bose that way, in hopes the tv audio comes through Bose, then hope my xbox 1 works on HDMI port 2 (if it doesnt come through optical cable connector) will report back with updated information if successful.
In addition to the arc input you have to refresh the hdmi-cec devices in the tv settings to get the audio working.