Just like the title says, occasionally, especially when in caves, important sounds don't play.
I've had this two times so far, a creeper would explode behind me without making the hissing sound, surprising me and almost killing me.
I also had this once when killing a zombie, the hurt sound wouldn't play on every hit (hit as in: mob takes damage) like it should.
Not sure what is causing all this to happen, lack of sound channels in the game? Soundcard issues (SB X-fi Titanium)?