Laptop or Desktop computer?
Have you tried hooking it into your headphone plug? You should get decent quality out of that. I am guessing you are using a 3.5mm headphone to RCA (white and red cable) converter to hook into the receiver.
As to the sound over WIFI, unless you have a specialized peice of hardware that streams the audio from your computer over the wireless network, then you aren't going to be able to do anything over WIFI. Further, the quality of this would be no better than straight from your computer.