I'm still using 10.6.8. The User Interface sounds, emptying trash etc, are gone. If I reboot they sounds come back for maybe a few hours to a day and then are gone again. So far I have deleted sound plist files and reset PRAM but no luck in fixing this issue.

Any ideas for permanent fix to the missing UI sounds?

This is something of a known issue, I have also been affected by it. Unless there's been a new development recently, nobody knows exactly what cause it or a reliable way to prevent it.

Not exactly the answer I was looking for but at least I know I'm not alone. Thanks. l'll keep my fingers crossed for some new development.

Kind of interesting that I've had this Mini for almost 3 years now and this behavior only started several months ago.