What would be the best way to setup input/output to my home theater system?

I have a home theater system in which I need to be able to coect atleast 3 inputs (all HDMI) to a projector. The issue is with audio. I have a 5.1 surround sound system (plaing to get 7.1 soon) which has 6 inputs, all of which are RCA cables (front left/right, side left/right, and sub/center). The issue here is that I have 2 devices which provide only stereo output. But a computer with a dedicated sound card as the third device. I have looked into AV recivers, however they do not seem to have 5.1 input. Only 5.1 (and more) output. The AV recivers I found had only stereo input for each device that can be coected. Am I not looking hard enough? Im pretty sure there is some solution to this issue. Also, I would not like to mess around with wires every time I would like to switch the source. Just the press of a button would be nice. Thanks for all your help.