If you just want the PC sound to come out of the Hi-Fi you can use a wire. If you want it to be wireless I saw in a little shop in lakeside a transmitter that you plug in to the PC soundcard and it then transmits to a recover at the other end. I think it ran on high frequency radio so it won't be ultra Hi-Fi but considering the rest of your kit (no offence) might not be to bad especially if you are only playing MP3. It was about £15!

As people have said, the squeezebox will only play MP3's or other music files, it wont just send any sound from the computer.