Extracting audio from MP4 video into MP3

"If you prefer a GUI, grab SUPER © (Simplified Universal Player Encoder & Renderer). It will do the job (and much more).

SUPER © is a GUI to FFmpeg, MEncoder,
MPlayer, x264, musepack, monkey's
audio, true audio, wavpack,
ffmpeg2theora and the theora/vorbis
RealProducer plugIn.

Drag & drop any audio file or even any video/audio file and extract the audio part by checking the Disable Video box, thereby converting it to:

aac
ac3
amr_nb
amr_wb
ape Monkey's Audio (Lossless)
flac FLAC (Lossless)
mmf
mp2
mp3
mpc
ogg FLAC (Lossless) or Vorbis High Quality
tta True Audio (Lossless)
wv WavPack (Lossless)
wav
wma

You can speed up the audio extraction by checking the Stream Copy box, which rips out the internal audio without conversion into another format.

SUPER © is freeware.

Note that SUPER comes bundled with OpenCandy adware. Mitigations:

Install in a virtual machine
Disable the network after the first step of the installation. The installer verifies that an Internet connection is present. After that you can disable it and ignore all warnings. I'm not entirely sure that this prevents all adware from being installed, though.
You can remove OpenCandy after installation: http://www.malwaredictionary.com/2011/06/opencandy/"

If you are on a Mac, a simpler method in the future may be to right-click on the the file in the Finder, choose "Get Info", and change the extension from .mp4 to .m4a. Then open it in iTunes and convert it to an mp3.

On Windows 7, Any Video Converter worked right out of the box for me. Looks like Mac is supported as well.

"If you perform a lot of audio editing, there's a reasonable chance you already have (and love!) Audacity.

It will open the mp4 file and allow you to ""Save as"" a number of file types, including mp3. It also provides a simple dialog for any parameters you might want to change, including things such as sampling rate and MP3 tags.

If you don't have Audacity, it might not be the most direct path, but it's far more versatile than a simple converter program. It's a fully functional audio editor, open source, and cross platform."