How to extract audio from mp4?

Demuxing is in fact the thing to look for. I can only guess that the OP may be using ffmpeg to reencode, not just demux (+ remux, given that raw aac isn't the most handy format either).

My little audio extraction batch file basically goes like this:
Code:
%path-to-ffmpeg%\ffmpeg -i %1 -acodec copy -vn %1-audio.mp4
 
yea, if you dont want to re-encode like sgrossklass says. you want demuxing. very easy for computers to do, with little cpu too. also no quality loss! 😀
more recent ffmpeg uses "-c" for codec chocies:
Code:
ffmpeg -i youvideo.mp4 -c:a copy -vn yourvideoaudio.m4a

.m4a is a container for audio like acc/ac3 audio formats, thats used in mp4 video container.