Sorry to break it to you: Once you lose information by reducing the quality of any recording (of any type) you can never get it back. ffmpeg can take an mpeg4 file and re-encode it to a dvd format but the resulting file will never be better then the source file. (unless you have magic powers.)
If you are really determined and can get a clean sample of noisy silence from the movie; you can strip the audio out and use audacity's noise removal feature, then multiplex it back together. Results will vary.
