Easy enough to do, I just implemented this for a customer's project.
On the OpenWRT box, you need to make sure you have all the necessary bits for USB sound (i.e. kernel module, core sound support, etc) that should all be described in the wiki/elsewhere. Make sure you have alsa-utils installed also to get the alsa programs. Once you've got your USB sound card hooked up and confirmed detected and working the next step is to get a copy of RTPTools:
http://www.cs.columbia.edu/irt/software … /download/
You'll have to cross-compile them yourself for openWRT as I don't believe there's a package made for them (could be wrong though!)... I cross-compiled them since I was building a custom image anyway.
Then all you have to do is install the rtptools onto your device, and use a simple command line to pipe the RTP to aplay:
rtpdump -F payload 224.0.0.56/6112 | aplay -r 44100 -c1 -f S16_BE
You'll have to adjust the multicast IP and port numbers of course... and you can tweak your aplay rate/channel/format settings to match whatever you're using to generate the audio feed. In my case I was using the RTP send module from PulseAudio, and configured it to send 44100 mono in BigEndian format.
Best part of course is it works like a charm using almost no processing power (the RTP stream is raw audio, so you don't need to process/decode it on the openWRT box). I tested it on an old 200mhZ Asus WL-500gP and playback was flawless.