You're probably looking for something like this:
http://www.nslu2-linux.org/wiki/HowTo/SlugAsAudioPlayer
NSLU is similar to the WRT54GL but it has a USB port.
http://www.compuplus.com/i-Linksys-NSLU2-NETWORK-STORAGE-LINK-USB-20-Easily…
Plus a USB Audio device in.
http://www.cables4pc.com/goods_detail.php?goodsIdx=142
or on ebay
NSLU can then stream music from your computer or Internet radio.
It goes out the USB port and converts to analog.
Use powered speakers since you don't have an amp.
Plug a USB wireless network adapter into the NSLU2 to go wireless.
There is a more expensive version of the WRT54GL that has a USB port.
Or you could just buy a SqueezeBox which what you are trying to build
http://www.slimdevices.com/pi_overview.html?
--
Jon Smirl
jonsmirl(a)gmail.com