Well, I bought a cheap USB-adapter via ebay (1€), plugged it in, killed waveserver, startet esd and got a good output using Winamp plugin (resampling) instantly. My adapter is started using 48kHz, 16Bit, 44,1kHz is not supported.

Then I did the following: kill esd, Winamp brought an error message because of this. OK
Copied esd to /opt/ altered post-boot, flashfs save/commit, reboot

So, since that time no more sound! Winamp doesn't complain about transfer errors, it keeps playing until I kill esd.
ESD produces CPU-loads of 1-15% when playing, so it does something.
Sound hardware is also working, because when unplugged or audio.o unloaded /dev/sound is unpopulated.
On my notebook running Vista, the adapter works perfectly!
Output keeps silent, when the output failed for the first time, I got an infinite beep with approx. 400Hz.

I restarted the router several times, started esd manually with different parameters, tired the other output plugin, ... but no result after spending more than two hours solving that problem.

I use a WL-500gP with Koppel v1.69 and mediatomb 0.11.0-r2 static mips32.

Does anybody have a recommendation for me?