Video2MP3 is a simple program which does exactly what its name suggests: give it a video, and it will give you an MP3 of the soundtrack.
The process isn't exactly difficult. Just drag and drop a video onto the program (MP4, FLV, 3GP, WEBM, MOV, AVI, WMV and other common formats are supported), click "Convert to MP3", and the soundtrack will be saved in the same folder as the source clip.
If you think this might be just a little too basic, then in some ways that's true. The program can't convert multiple files in a single operation, for instance. And there's no control over how your MP3's are encoded.
It is very easy to use, though, and if you only want to convert single files very occasionally then Video2MP3 might be good enough.