Apologies if I got the chorus of a certain hit Youtube song in your head, but let's be honest, the same premise of that song certainly applies to koalas. After all, you probably never heard one of them make noise before.
But now that you've seen this video, you might better understand why we never hear koalas speak -because they are embarassed by their shockingly deep voices. If you looked like that and sounded more like a pig than a cute fluffly bear, you'd probably be shy about your noises too.
Via Laughing Squid