Soundcards/RTL8139/USB won't work in a stable way until Michael Boehmer releases his updated firmware. And he is very busy with other projects so it's low-priority.
I think that there are catweasel3 drivers for Prometheus but I don't know if they work.
The only SoundCard I know that could work with Prometheus if a driver existed is Terratec 128i, as it doesn't require DMA. If anyone has one and a CSPPC & Prometheus it could be interesting to try out MorphOS (I think it will probably work, RTL8029 seems to be stable).
If anyone is thinking about buying a Prometheus just buy it for RTL8029 and Virge/Permedia2/Voodoo3/4/5 (a Voodoo3 will be the best one). Don't buy it expecting future drivers because the situation hasn't changed since it was released and we are not sure about future drivers appearing or working correctly.
If you want one think about it as a Voodoo3+RTL8029 card. If new drivers appear it would be a total surprise. I would be very happy to hear that Michael Boehmer releases a new firmware but I don't expect it happening before 2008 as he has more interesting projects that no longer sold old PCI buses.