Noise reduction with ffmpeg

[xxlray]

I archived some old VHS tapes using an stk1160 video grabber. Naturally these are a bit noisy and I read that ffmpeg can do noise reduction in two pass mode. This is what I tried:

ffmpeg -i invideo.avi -sameq -vcodec libx264 -nr 1000 -pass 1 -passlogfile passlog -acodec libvo_aacenc outvideo.mp4
ffmpeg -i invideo.avi -sameq -vcodec libx264 -nr 1000 -pass 2 -passlogfile passlog -acodec libvo_aacenc outvideo.mp4

Now I have different issues:

1. Audio is gone after the first pass
2. The first pass result looks fragmented and a lot of frames seem to be missing
3. Second pass does not start and gives the following error message:

[libx264 @ 0x1630d60] constant rate-factor is incompatible with 2pass.