The optical output of most, if not all TVs will NOT pass multichannel audio from an external source. It will turn everything into 2-channel LPCM. The only multichannel you can get from a TV's optical output is from its own tuners.
This is stupid, of course, because otherwise we could all be using our expensive new TVs as the sophisticated AV switchers that actually are.
But until you get an HDMI AVR you will need to connect your digital audio outputs directly to your existing AVR to get multichannel sound.