i am having the same issue.. is there a fix?
Can you confirm that you have the latest version of Todo or Todo Cloud installed?
You can update to the latest version by visiting: https://itunes.apple.com/us/app/todo-cloud-to-do-list-task/id568428364?mt=8
Are you using the standalone version of Todo, or Todo Cloud 10?
If you're using Todo Cloud 10, you and your alerts are no longer working, you'll need to have the reminder alert removed then re-added to the task.
Todo Cloud uses the default notification tone in iOS. Can you confirm if Todo is enabled with Notification centre within your iOS device settings?