Hello, you can do this fairly simply with VLC media player. If you don't already have it, it is a very functional media player available for windows and Mac. You can download either version directly from CCM.
Open VLC and follow these steps
Click on Media > Convert/Save [CTRL + R].
Click on Add.
Browse and open your video file.
Click on Convert/Save.
Under Settings, in Profile choose Audio – MP3.
Hit Browse and give a destination file name.
There is also information in this CCM article on how to extract audio from any video file using VLC.