1] Pazera Audio Extractor, excellent free software from Poland. Extracting audio from a video file is very easy:
The Pazera Free Audio Extractor interface is very simple and user-friendly. To extract audio tracks from video files, just drag & drop them into main window, select proper profile from list, and click the CONVERT button. If the audio quality is poor, increase audio bitrate.
2] Audacity, a powerful Open Source program which can do many useful things with audio files.
Here is a flowchart I have made which explains what you need to do: