Install the Mplayer binaries for your operating system from http://www.mplayerhq.hu/
Command line option:
mencoder -ovc frameno -oac mp3lame -lameopts cbr:br=128 -of rawaudio -o <output_file.mp3> -audiofile <input_file> <dummy_video_file>
Make sure that the play time of the video file is more than the play time of the audio you are trying to convert. In case it is not, you can give multiple video files in the place of one.