[Ken Case]

OmniFocus is not a calendar, but it does have sounds and alerts and calendar integration.

In OmniFocus for iPhone, go to Settings->Notifications->Sounds & Alerts. On that screen you can enable Due Reminders (with independent settings for showing alerts and playing sounds), or turn on Calendar Alarms which you can subscribe to from the iPhone's built-in Calendar app to get snoozable reminders.

Hope this helps!