The recently-released (May 14th) SP1 fixed this problem for me. Install it
and give it a try. FYI - installing SP1 was easy but at first it appeared
that the problem wasn't fixed. I rebooted just to see if would make a
difference and when it came back up all the notification sounds were there
so you may need to reboot after installing SP1 to fix this particular
problem.
Hope that works for you.
