Those iRV 66s can be real frustrating.
Bob's Manuals - Download PDF Owner's Manuals
if you don't have a manual, you can get a .pdf and download or print one from that site.
As you read through it you should notice that the unit can be rebooted from the power switch, just like a laptop.
Top 3 buttons on the unit are speaker on/off buttons.
SRC button controls the source audio. ie. TV audio means the source is the RCA plugs on the back of your TV set connected by cable to iRV.
I think there is one that says DCC/OIC which is what is used if you connect the two with an optical cable.
This SRC setting could be your root cause
Hope that helps.