I saw something at bestbuy and circuit city that allows you to send your audio from a computer to any stereo in your house wirelessly. I don't remember what it was called but it was in the pc section of the stores.

optical should work fine for your sound assuming you have the inputs/outputs available. Also you could get a receiver that lets you plug everything into it and run less cables to the projector. Meaning your hdmi going to the receiver would pickup the sound and output the video then to the projector over hdmi without the need for a seperate optical cable. Plus you could have multiple hdmi's going to receiver with only one needed output to projector.

get a sound card for your pc that has the outputs you want/need.

As for receivers...most new receivers have multiple digital inputs. Just search around and find one in your budget that has what you need and people recommend.