a nice working freeware Video MP3 Extractor
I looked at the website for the tool, and I did not see it list supporting mp4 files. The process I use for extracting audio from video files is handy that it work for any initial video format and requires two freeware tools.
First, I convert the initial video file (mp4 in this case) to AVI with the great free opensource transcoder Super
Second, I take the new AVI file and open it with the great video tool, VirtualDub. Once you have opened the file in VirtualDub, you can export the sound as a wav file by clicking "File" -> "Save Wav...". Then you have the wav!
Hope that helps,