In case anybody Googles the WL-500W and hits this thread, I was able to stream audio on mine using the latest Backfire brcm-2.4 target. I ended up building my own although you could probably use the openwrt.org binaries as well.
You must load the "kmod-brcm-wl-mimo" driver to get wireless working on the 500W.
You must load the "kmod-usb-uhci," "kmod-usb-uhci-iv" and "kmod-usb2" drivers to get USB working on the 500W. This platform doesn't need the "kmod-usb-ohci" driver.
I ended up having to use ALSA, so load "kmod-alsa" and you'll find "alsa-utils" and "alsa-lib" useful.
"kmod-usb-audio" and "kmod-alsa" seem to conflict on the brcm-2.4 target. I didn't unload "kmod-usb-audio," I just commented out the "audio" invocation in /etc/modules.d/60-usb-audio and that allowed all programs that default to ALSA to work again. I believe OSS programs will work fine as well, since /dev/sound/* is populated as expected.
The version of MPD built in latest Backfire still causes trouble; it doesn't handle HTTP MP3 streaming correctly despite listing support for both in "mpd --version." So I had to use the following command to stream audio from my Squeezebox Server:
<squeezebox server>:9000/stream.mp3 -O -| madplay -
No remote volume control or player synchronization but this is better than nothing.