Someone wanted to know how to do this on another forum, so I dug around and found it.

  1. copy /dbdata/databases/ to /dbdata/databases/
  2. copy /dbdata/databases/ to /sdcard/calendar.db
  3. open /sdcard/calendar.db
  4. Open Events, scroll around and find the reminder you want., then look all the way to the left column for that line, you will find the event_id field, note the data in that field. Hit back
  5. Open Reminders, find the event_ID from the previous step, tap the line and choose edit. Edit the data in the Ringtone field with the full path to the sound you want to use, i.e. "/sdcard/mysounds/calendar.ogg" (dont type the quotes)
  6. Hit back until you are out of the SQLLiteEditor
  7. copy /sdcard/calendar.db to /dbdata/databases/