Default No sound on Calendar event WM6.5

Hi,
I have a problem with my original rom WM6.5 .
Often I haven't any sound notification from calendar. I try to search register flag on wm registry but the problem was the same .
I discover only one solution to this problem...make a MASTER CLEAR and after the calendar return to notificate the event with sound .
So I have to restore my contact, message...and so on, but I haven't make this about every month!!!
There is a solution to this?

thanks.