I'm on the minecraft wiki's ambience page. Only thing that sounds sorta like what me and my friend heard, is that guitar chord. But it does NOT sound like any guitar I've heard. It was more laughing and crying mixed together. Very creepy. I couldn't recognize it from any creatures in the game and I've heard pretty much all of the random sounds.
And the only cave nearby was long ways off, all lit too. I didn't get the details from my friend, when he heard it but I was going up a hill, getting chickens when I heard it. Near a river between the hills. Next time I'll run fraps and hope to record the sound when I hear it again.