That can be done in a number of ways; with Quicktime it is more or less immediate: Window, movie properties, select audio, extract, File, export.
But I wanted to do it without recompressing, since the audio was already in MPEG4 format (ACC).
My first impulse was to try mencoder, but it doesn't seem to work, at least not in a few quick tries. But that can be because the version I have installed is somewhat b0rken, or SVN-ish, or macports-ish, or because the output options say they are beta, or because I should re-study the manpage. Again.
So while Macports installed my second impulse (ffmpeg), I went back to Quicktime to make sure that there is not some option for "direct", "non-reencoding", "codec: copy" or some such.
And there is! But it is somehow misleadingly buried. You have to:
- select MPEG4 exporting in any of its incarnations (which isn't a good start because selecting already makes you feel like you are modifying things when you don't want to),
- ignore the presets (which is where I most expected to find such an option),
- pretend you are selecting options to re-encode (which in my case was exactly what I didn't want to do, so at this point I had already given up and trying to reencode with minimal loss), and ...
- there it was: the format listbox in the audio tab includes the option to just use the already compressed audio. And it seems to do what it promises, since the export is pretty much instantaneous.