Your setup is kind of unusual. TVs don't tend to have a way of playing 5.1 themselves. Your optical connection from TV to receiver can carry 5.1, but only if it is compressed with something like Dolby Digital, as RDJ mentioned. Optical can't carry a full resolution 5.1 signal. This means that you'll need to have MC (or something else) handle that conversion. Note that Dolby Digital is lossy, so you are necessarily going to lose audio information and quality. How much loss is a matter of opinion. Dolby Digital is on a lot of DVDs and people didn't complain too much. But it's not exactly "hi-fi".
It would be a more pure signal path if you could connect the HDMI directly to your receiver. Or perhaps use an inexpensive 5.1 (or 7.1) channel DAC, so you can feed a nice full resolution signal to the DAC (or receiver via HDMI) and get everything your 5.1 files can offer. In the case of an external DAC, you would, of course, need a multi-channel analog input on your receiver.
Brian.