Check the settings on the box. I don't know what they are off hand, but sometimes pcm and another, the wrong one often doesn't work. Also check the settings on your amp, If it's like my Onkyo I have to assign the correct digital input to the appropriate video signal.
Thanks for getting back to us, I'm, sorry again for the problems you're currently experiencing.
It sounds like there could be an issue with the optical out on the TiVo as you've checked the cables and the sound system itself. With that in mind, I'd like to arrange for an engineer to come and take a closer look, if you're available?
I've sent you information regarding this in a private message, which you can view by clicking the envelope on the top right of this page.