I know that I'm my AVR, if you have audio coming into the optical input and the video coming into an HDMI port, you need to tell the AVR where to pull the audio from in that HDMI port's setting..See if you can find that setting and change it.
Hmm, what is the model of the AVR?
Remember that the Roku doesn't support Dolby Digital, only Dolby Digital Plus, and DD+ can only be passed thru HDMI. Make sure your Roku is set to stereo/PCM (sorry, I don't have the exact setting in front of me as I'm at work) not DD+, because I'm guessing if your AVR has not HDMI that it doesn't support DD+.
Cable works, can see red light
Optical output from TV into the optical input on the AVR, yes.
Red light glowing from TV optical output, yes.
Once plugged in to the TV, the other end of the cable glows red.
I appreciate mentioning the possibility of a cap, that seems to be a common issue that's infrequently answered for other people, having gone thru many threads while researching this... that said - no, the cap is not on.
Cable clicks into place on both the TV and the AVR, yes.