I have been having the same problem. Outlook 2011 sounds disappear every time my computer goes to sleep. Quitting and restarting Outlook does not solve the problem, but If I do a shut-dwon and/or restart the sounds come back again, until the computer goes to sleep again and the cycle restarts. It is very annoying to restart the computer in order to get the sound back. This has been a persistent problem now for many months.