Command to extract audio from video without conversion

Command line

If you're willing to forgo using a GUI you can use ffmpeg fairly easily to do this.

Sample file

If you go to QuickTime: Sample files, you can download this sample file, sample_mpeg4.mp4. After downloading it, unzip it.

$ ls -l | grep sample
-rw-rw-r-- 1 saml saml   235829 Nov  4  2005 sample_mpeg4.mp4.zip
-rw-r--r-- 1 saml saml   245779 Nov  3  2005 sample_mpeg4.mp4

Example

You can extract the AAC audio frpm the mP4 file.

$ ffmpeg -i sample_mpeg4.mp4 -vn -acodec copy sample_mpeg4.aac
FFmpeg version 0.6.3-rpmfusion, Copyright (c) 2000-2010 the FFmpeg developers
  built on May  5 2011 19:20:01 with gcc 4.5.1 20100924 (Red Hat 4.5.1-4)
...
...
Output #0, adts, to 'sample_mpeg4.aac':
  Metadata:
    encoder         : Lavf52.64.2
    Stream #0.0(eng): Audio: aac, 32000 Hz, stereo, 48 kb/s
Stream mapping:
  Stream #0.0 -> #0.0
Press [q] to stop encoding
size=      31kB time=4.99 bitrate=  50.4kbits/s    
video:0kB audio:30kB global headers:0kB muxing overhead 3.594943%

The resulting AAC file.

$ ls -l |grep sample
-rw-rw-r-- 1 saml saml    31468 Oct  4 22:09 sample_mpeg4.aac
-rw-r--r-- 1 saml saml   245779 Nov  3  2005 sample_mpeg4.mp4
-rw-rw-r-- 1 saml saml   235829 Nov  4  2005 sample_mpeg4.mp4.zip

You can also use ffmpeg to confirm the file format:

$ ffmpeg -i sample_mpeg4.aac 
...
...
Input #0, aac, from 'sample_mpeg4.aac':
  Duration: 00:00:05.59, bitrate: 45 kb/s
    Stream #0.0: Audio: aac, 32000 Hz, stereo, s16, 45 kb/s
At least one output file must be specified

GUI

You can use VLC and it's Convert/Stream feature to do this. After launching VLC.

Example

Select Convert/Save from File pulldown

Select video file, and convert stream

Start the conversion

ffmpeg should do this for you, without re-encoding:

ffmpeg -i in-file.opus -c:a copy out-file.caf

That works here, but I don't have a Mac OS X box to test the resulting file on.

PS: You really ought not convert between lossy formats. Losses always get worse.