Make sure:
You have line out on the computer, not headphones or speaker
You have it going with a stereo (TRS) 1/8 inch plug to two RCA plugs
The RCA plugs go into a line level input on the receiver (not phono)
Your cables are no longer than about 10 feet (longer than that and you should balance them for which you would need pro gear)
Your gain (volume) is not turned too high (switch to the appropriate input with the volume on infinity or zero and, with something being output from the computer, slowly bring it up until it sounds right).
Other than that, I don't know. I use a Power Mac G4 and I don't know how good the eMac line out quality is-and besides that I have a pro sound card so it wouldn't matter. Good luck.