How can I demux avi?
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