That would be a box which accepts digital audio over
ethernet and outputs
analog audio. Something like a small computer running netjack...
I should note that if you are game to do audio computing using a small
computer, the WRT might not be your best option, but there are other
ways!
For example, you could put together a small-form-factor (mini-ITX or
such) computer that has an audio or SPDIF output on it. Take a look
at the AudioPint project for example.
http://audiopint.org/
Or to go _really_ small, have a look at gumstix:
http://gumstix.com/
A gumstix + roboaudio board will cost about $200 and it can do audio
output directly. Though I think you'll have the same issues in terms
of floating point operations.
There are likely several other options for small computing.
Steve