Whenever I loged on Facebook on Safari 4, I had to leave chat signed off. Whenever I received a message, I got an immediate error message saying something about plugins and a "MIME" file. I've googled it  and now i have found the remedy for it....Safari is a great web browser and I love it..- Goto Options from facebook chat menu- Then unclick play popup sound...It  really works and this is only for safari users........