ffmpeg AAC "Can not resample 6 channels..."

When you try to encode with ffmpeg and you end up with such an error

Resampling with input channels greater than 2 unsupported.
Can not resample 6 channels @ 48000 Hz to 6 channels @ 48000

you are probably trying to encode from AAC with 5.1 audio to less than 6 channels or different audio sampling rate.

There are three solutions:

1. As a solution either do not reduce the audio channels and change the audio sampling rate or do convert the audio with faad first.
2. Apply one of the available ffmpeg patches to fix the AAC 6 channel issue...
3. Split video and audio and convert audio separately.

The third solution can be done as following:

1. Extract audio with ffmpeg:
   

   ffmpeg -y -i source.avi -acodec copy source.6.aac

2. Convert audio with faad:
   

   faad -d -o source.2.aac source.6.aac

3. Merge video and audio again with ffmpeg:
   

   ffmpeg -y -i source.avi -i source.2.aac -map 0:0 -map 1:0 -vcodec copy -acodec copy output.avi