Shoutbox

I'm running on Vista with Windows Live Messenger 8.5
Whenever a contact signs in, I receive mail, or I receive a new message, I sometimes here the sound notification, sometimes not.  This is particularly annoying since it makes me tend to miss messages. 

I've already checked under Tools > Options > Alerts and Sounds.  Everything is checked off and each of the sounds works.   I've also tried uninstalling then reinstalling.  I've also already removed Plus! but the problem persists.
I can't seem to find a pattern as to when the sounds play or not, can anyone help me out?

Thanks in advanced

I get this to sometimes. Mostly just when people sign in. All the other sounds seem to work fine.

does anyone have a fix for this?