5.1 setup not playing through HDMI

I have a 5.1 speaker system:

LG LHB953 Network Blu-ray Disc 5.1-Channel Home Theater System

and a 40' LCD:

TOSHIBA 40" 16:9 8ms 1080p LCD HDTV 40RV525R

I have an hdmi cable running from the tv to the receiver and that is it. When playing a bluray, the sound works fine, but then when i switch the input from hdmi 1 (where the bluray/receiver is plugged in to) to ant/cable to watch regular cable tv, the sound only comes from the tv speakers.

Any ideas on how to get regular cable to output sound through the surround sound speakers?

Thanks

You would need to connect a sound output from the TV (optical digital or analog red, white cables) to an audio input on the receiver (if it has one).

How do you receive cable TV? Do you have a set top box or does the tuning happen in your TV? You will need to feed audio from the device doing the tuning to your receiver.