Mobs get quieter as I turn away from them, and if I'm facing a direction more than 90 degrees off, they are totally silent. In other words, I have a 180-degree deaf zone behind me. So when a creeper comes up behind me, I can't hear the fuse-burning sound, making it pointless for that sound to exist. And pointless to play Minecraft, since I'm just going to keep getting blown up by unavoidable creepers. Even the creeper's explosion was silent.
This also silences sounds from other things, like lava and water.
This bug makes Minecraft unplayable.