I'm using a HTC HD mini with WM 6.5.3.
It seems, there is a bug regarding the reminders of calendar an tasks: When there are two or more events coming up at the same time (eg two tasks with reminder time 9:00), no reminder sound is being played. If there's only one event, I can hear the sound. The other behavior (message, LED, etc) is normal.
(Before the HTC HD mini I had a SE X1 with WM 6.1, which does not have this "bug".)
Can somebody try to reproduce this behavior, please?