First of all, the DVD Navigator works quite a bit differently compared to how any other source filters work, and because of this playback speed adjustments are also different.
Secondly, if by "Rewind" you actually mean playing back backwards, that will never work, and you should forget about it (it also doesn't work with any "normal" file). If you don't mean this, then use a better term.
Fast Forward is also a problem, depending on how its handled.
2x is usually not a big issue, but at 4x or even higher, you get into trouble, because there is more frames to be decoded then the decoder and/or renderer is designed to process, so something has to start dropping them.
That said, LAV Video does not support changing DVD playback speed in 0.55, but it will in future versions.
I'm not sure i ever saw audio continue on a DVD in fast forward.
PS:
Personally, i think changing playback speed is a nonsense feature, if you want to go forward, seek forward, don't watch through faster images. :p