Minecraft audio has many implementation issues we could get into, but this one is fairly new and quite disastrous.

If a source of audio is behind the player, it will become so quiet as to be nearly impossible to hear, even if the sound source is inches away. This has to do with the pan law implemented in the game, that is, how volume is raised or lowered as you pan, or in the case of minecraft, when the player turns left or right. If the player turns around so far such that a sound source is directly behind them, the volume of that particular sound is lowered too drastically as to become almost inaudible.

The problem is benign if the sound source is a jukebox. It becomes odd when quickly placing blocks and torches and having sounds fade in and out as you turn back and forth. It's a disaster when exploring, and monsters, who would normally give you a slight chance to detect them through their ambient noises, can no longer be heard at all when they're behind you.

I'm not sure why this was changed, but it seems to have been relatively recent. As a sound designer and general audio person, I find it particularly frustrating, so I hope you'll excuse my passion!