As I know you can do this operation even online. But I recommend to install desktop version of some popular application to have full functionality. My recommendation is SuperSpeed Video Converter. Besides convertion, you can also edit your videos – crop, rotate, stabilize, add subtitles and watermarks, delete sections, and more. Very useful tool! Here is a link for you
www.videoconverter.com/how-to-convert-webm-to-mp3.html