Like said in another thread I'm testing some avi muxings but being lazy I'd rather not have to encode my DVDs again to xvid. So I was wondering if it's possible to adjust the speed of the AC3 streams I already extracted from my original PAL DVDs to match my xvids on 23.98fps. When I mux them without any adjustment my german audio stream is too fast for the video.

I guess I'd have to go from AC3 to wav first but even so, how do I slow them down by 4,27%?

Use BeLight or Audacity.

quality, simplicity wise it would be better to simply re-rip/reencode the 25fps dvd (that way you can keep the original ac3 ...).

You can change the frame rate of AVI files without re-encoding the video. VirtualDub(Mod) can do this if you save using direct stream copy.