Not Receiving Mail Notifications

For some reason, I have recently not been receiving the audio notification when I get a new email. I say "recently" because I have been using Apple Mail for years now and it has always worked, but I think it stopped working when I upgraded to Snow Leopard. The really odd thing is that sometimes it does work. There seems to be no rhyme or reason for when it does or does not work, except that about 90% of the time I get no sound. Has anyone else had this issue, or does anyone have any idea how I might fix it?

I can remember I had a the same problem, although I can't remember if disabling the IDLE command fixed it or if an update did.

Disabling the IDLE command and other things to try are in this article.
Mail not playing the new mail sound | MacFixIt - CNET Reviews