Ok, there is definitely something strange going around.
The 550mb async movie file is on my device and still async. I can't edit it in virtuadub, because the audio gets complete out of sync after that. The original file is 200km away, so I called my dad and instrcuted him to encode a 30sec preview in media converter. He just send me the file and I tried on my n800 and it's perfectly synced. Same file, same settings. The one fully encoded, the other one just with the 30sec option. Strange? In the meanwhile I encoded about 10 different files and all work well. So there, on the other computer (200km away) should be the other two full length movies which were out of sync. I guess the problem is not related to mplayer after all, but to media converter? Nevertheless the async one is synced in my windows machine and I definitely can't cut it with virtuadub.