I'm guessing that app hooks into calendar events, but that involves writing code.
This works for me:
1. Root your phone. The easiest way I've found is located
here.
2. Using Root Explorer (or similar file management app that has been granted SuperUser privileges after rooting) Move or Copy your notification sound file(s) to:
/system/media/audio/notifications/
I changed the permissions to rw-r--r-- for the moved files, but it turns out their default permissions also worked fine.
3. Back in the Calendar app, select your moved/copied notification ringtone from the list.
4. Unroot your phone, if desired.
Doing the above allowed me to use custom tones for the Calendar notifications - I tested with .ogg files, but perhaps .mp3 or others may also work.
(At first, it seemed a side-effect caused loss of system sounds, but it turned out I somehow turned the System sound volume to 0 through some fumble fingering.)
So, no side-effects and I'm back to being stock, unrooted. Whole process took less than 10 minutes with testing.
- ooofest