Tank you very much for your answers.
As far as I understood ffmpeg is of no help because it is build to change the frame rate without altering the length (deinterlace, pull-up/down or simply dropping frames). I just want to revert the speed up - so changing length is intended. But It took me months to get mencoder to work for me - so maybe useful options of ffmpeg are still hiding from me ...
Taking your infos into account, I'll think about trying two things:
1. Using windows-aften with beSweet as decribed in your post (although I still wonder why my ears won't find a problem with ac3enc.dll, maybe because I use a high bitrate like 448 anyway?)
2. I found out that sox has a promising option called "stretch" which claims to alter the lenght of an audio file without changing the pitch - this - together with aften - maybe will do the job I'm running nowadays with beSweet even native on linux?