Confirmed this on SSP and on SMP.

Whenever I hear one of those "scary" cave sounds (http://www.minecraftwiki.net/wiki/Ambience), they sort of get chopped off before they fade away, killing the scary aspect of the sound. it happens with almost every one of them that goes on for a bit, like Cave 5 in this video: http://www.youtube.com/watch?v=LwcgG7ZYlFE

Not much more to say about the issue.