The optical out on tvs, is for getting HD channel's 5.1 surround, from over the air (antenna). It's not for doing a daisy chain of a signal.
You may also need to go into the cable box setup menus, and set the audio output to Dolby Digital. Understand though, that the only channels that broadcast in 5.1 are HD channels, premium movie channels (HBO, Starz, etc.), and some PPV channels.
When you get a new receiver, I'd suggest one of the new Onkyos (TX-SR605 and up). They can process even the new HD audio formats over HDMI, so they'll do what you want. Especially if you're ever going to get a Blu ray player. Can also look at Yamaha RX-V663................
