Static from 5.1 signal

[ST33]

I get static from my home theatre speakers when watching a TV show with 5.1 sound.

I have my cable box connected directly to my Yamaha stereo receiver via TOSLINK fiber optic cable. When the cable audio signal is merely stereo, the sound comes out of my Yamaha's speakers just fine. But when the program switches to 5.1 (i.e. on an HBO HD show) the static starts up.

I've tried two different cables, I've tried both the TOSLINK inputs on my Yamaha, and I've tried two different cable boxes.

What else could cause this?

 

[ST33]

Cable box is a Motorola DCH3416
Stereo receiver is: Yamaha HTR-6050

 

[ST33]

This is driving me nuts.
Could this be a software issue?
Do I need to change some setting in my cable box??