I personally like to use the tcextract tool from transcode to demultiplex, and mplex to recombine them, but then again, I mostly create mpeg2 videos for DVD's. There are other ways as well, such as mplayer's -dumpaudio option.
But you can even use ffmpeg with "-vn" and "-acodec copy" options to rip out the stream. You may have to pay attention to which output format (-f) you use though, depending on what the source file is (mp3, ogg, etc).
Or even better, use "-acodec wav" to convert the audio directly to .wav format, again while using the -vn option to make it ignore the video stream.
Then you can use faac to convert the audio to aac, and finally combine the streams again with ffmpeg.