Use Free Video to MP3 Converter to extract audio from video files (*.avi, *mpg, *.mpg, *.mp4, *.wmv, *.asf, *.mov, *.qt, *.3gp, *.3g2 and *.flv) and save it as MP3.

You can convert the whole file to MP3 or just a partition of a movie. It's clearly free. No adware or spyware. It's safe to install and run.