I also ran tests with a different movie which uses the Indeo codecs (native) for decoding. This one runs smoothly (with audio AND video). In my opinion, this is additional evidence that the winemp3 codec is at fault (especially since my sample movie uses a not-installed DivX video codec which therefore does not consume any CPU, as it is running in audio-only mode). The native Indeo codec also does not crash on seek.
this still doesn't prove that the decoder is to blame (it may well be). it shows that the upper layers of wine work "correctly" with indeo decoder. Moreover, the point I question the most is the way the decoder (mpglib) is integrated into winemp3, which is potentially the core of the issue (if any) A+