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@xahlee try ffmpeg -f mp4 -i x.mp4 -map 0:1 -c:a copy x.mpa

-map 0:1 means stream number 1 from file number 0, which would usually be the audio track (edit as necessary). specifying a map at all disables the default map which would be to copy every stream, so you get only the audio, not both. -c:a copy means just copy it, no re-encoding. You may also want to specify a -f for the output to choose a container fomat; I am not sure what it will use by default, probably the same as the input.