Sometimes, playing 'ambient.cave' using the '/playsound' command plays a weird glitched-out sound instead of the usual cave noises. It only occurs rarely. I'd say about 1/10th of the time.
To give an example, I recorded it. The sound can be heard in this video at 0:14: https://www.youtube.com/watch?v=N0-rsYeqCB0
Command: /playsound ambient.cave master @a