I've never seen a calendar app that allows you to set a custom notification tone for an individual event. Business Calendar 2 has the option to set custom notifications for different calendars, so you could use that and create different calendars for the events you want to have different tones ("Birthdays", "Important" or whatever) and assign the corresponding events to those calendars. That's the closest I've found (I have checked aCalendar+, DigiCal and the Google Calendar app in addition).