Open Finder on your Mac
Navigate to the video file you want to convert
Right-click on the file
Select "Encode Selected Video Files" from the drop-down menu
Under the "Settings" heading, select "Audio Only" from the drop-down menu
Select "Continue"
A separate MP4 file will be created, and loaded in the same place as your video file. Note this doesn't delete or alter your video file in any way. This method can be used to convert any video file type to an audio file – but you don't get to select the type of audio file. It will always be MP4.