Bonus: Tip for Firefox Users
If you’re getting WEBM audio files instead of MP4 from YouTube (webm can only be played in a browser) , disable your browser’s WEBM audio support, then YouTube will send the audio in MPEG4 format.
Type “about:config” in the address bar, press
Enter, click through any warning message and search for “webm”.
Find and double-click
media.mediasource.webm.audio.enabled. Its value will change to
false and you’re set.