If you don't already have it on your computer, download and install the VLC media player. Free and open source, VLC is as powerful as it is complete and will not only allow you to play almost all multimedia file formats, but also to perform a multitude of operations.
To extract the audio track from a video in VLC, you must convert it. To do this, click on the Media (or File) menu, then choose Convert/Save.
The Conversion Setup window will appear. In the Profile line, click on the drop-down menu, and select the Audio-MP3 output format.
A blue bar indicating the progress of the conversion is displayed in the VLC timeline and disappears when the conversion is finished.