This bug has been referenced in a few device's threads here on XDA :
and also mentioned a lot on Internet.
It occurs in WM 6.1 , builds >20000 and <21058 (not sure regarding the latter).
It is confirmed to exist in builds 21054 and 21056.
As I myself do use that kind of reminders, and this bug bugged me a lot,
I did some research, and wrote small App , that provides a workaround to the bug.
In general - my App monitors the registry, and when the sound should be played for event, it plays it (same sound you chose for Reminders).
I hope that people who were looking for that solution still need it .
Known limitations :
1.For some reason , my workaround only works with WAV files set as sound for reminder. WMA files don't work, and I didn't manage to overcome this.If you have an idea - drop me a PM.
2.If you set the time out (Power Options) to 10 seconds - sound won't be played, due to some sleeps I used.
3.As the bug only affects the "all day" events, you might get your sound played twice for a non "all day" events. Actually , my program repeats the sound every 10 seconds until user performs "Snooze" or "Dismiss". There is a workaround for this one. Set the desired WAV file as sound for the Reminders, press OK.Then return to Reminders Notification , and uncheck the "Play Sound" square. This way, my application will be the only one to play sounds, for all the reminders.
4.This initial version doesn't treat the Vibration absence. I think it can be fixed easily though.
PS : In case of an interest, I might publish the 10 lines of the source code.
I would be really happy to get some feedback on this, as it is my first program,that goes to a such great (not only in numbers regard ) audience.