Unfortunately Avisynth only supports uncompressed audio. This issue is being worked on however (not by me of course).
But it is simple to work around if you have some extra space. Use VirtualDub to decode the audio (audio->full processing, file->save wav). Then put this in an avs file:
Code:
AudioDub(AVISource("video.avi"),WAVSource("audio.wav"))
Then you can use AssumeFPS and ResampleAudio easily.