When you get new messages, emails, Skype chats, and other routine things that should elicit an alert sound from your Mac, but those sounds never happen, that can be just a little annoying, especially if you rely on those audible alerts to let you know someone is trying to chat. There's a good chance those alert sounds are just backed up waiting to come out, and while Apple needs to release a fix for the problem, the workaround is drop dead simple.
No audio alerts? The culprit may be your Mac's external speaker connection.
If you find that your Mac stops spewing out alert sounds, you're probably using external speakers. Just unplug the audio cable, wait a couple seconds, and then plug it back in. When you unplug the cable, your Mac will spit out a cacophony of alert sounds that have been stacked up waiting their turn to play, but once that's done, you'll hear alert sounds again the way they were intended: When they happen.
I found I typically have this problem on my MacBook Pro after it's been asleep. My audio alerts work just fine before it goes to sleep, but clog up until I pull out my audio cable after my Mac wakes up. It hasn't ever been an issue for me when I use just the built-in speakers in my MacBook Pro.
Hopefully Apple will address the problem with an upcoming OS X update, but considering it's been an issue for a long time, I'm not holding my breath.