I want to demux the audio from avi files, so that the files are lossless. I tried both Yamb and Ffmpeg. Both produce files less than 1 MB (for a video of 80 MB), which are unplayable through VLC. I tried avidemux, and it says it can't export in copy mode.
Your audio is PCM, but you're trying to streamcopy to AAC, which won't work.
To extract the audio to WAV, use
ffmpeg -i DSCN5008.AVI -acodec copy -y a.wav
To transcode the audio to AAC, use
ffmpeg -i DSCN5008.AVI -acodec aac -y a.aac
Note that AAC isn't lossless. But FLAC is a lossless and compressed codec.
ffmpeg -i DSCN5008.AVI -acodec flac -y a.flac