I've tested some USB sound cards on it and they both work.
A Zalman ZM-RSSC works fine but is for about 40€
A Kinyo DA-10 Usb audio converter costs 12€ and also works fine.
I have also tested some very cheap USB speakers, they were detected but the plugin crashes and I have to reboot the wl500g.
I haven't been able to find any info regarding the waveserver and waveservermain deamons used for the audio feature and I would shurely like to know what kind of stream it is. But I have the impression the there is not alot of people on the forum interested in the audio feature.
Anyway I am. Please find my comments here (the last posting on that thread): http://wl500g.info/showthread.php?t=1670&page=2