Why alarms in "To-do" applications are shown as notification
Astrid has this functionality. Add a task by giving it a title and then clicking ADD A DUE DATE if you wish. This opens a page where you can tap a due date + time. Click OK after making the necessary changes. Click the blue + sign on the right to add the task to your list.

Now tap the task to edit it - Scroll down if you need to and select DETAILS and then REMIND WHEN DUE &amp; OVERDUE

Choose the options as you see fit and ensure that the RING/VIBRATE TYPE is set to UNTIL I DISMISS ALARM.

Click ADD AN ALARM and choose when you want to be reminded.

To avoid this rigmarole in the future, you can choose to set by default that reminders should ring constantly until you dismiss them. Click SETTINGS > NEW TASK DEFAULTS > DEFAULT RING/VIBRATE and then select UNTIL I DISMISS ALARM

Also see REMINDER SETTINGS where you can choose a ringtone of choice.