Apple Reminders make no sound
This same issue drove me mad, not to mention almost making me miss an appointment, until I found the answer by accident. There are two places where the option to set the sound for a reminder appears (no, me neither).  The obvious one is:

Settings>Notifications>Reminders (allow notifications)>Sounds

Unfortunately, this is the wrong one.  The right one is in:

Settings>Sounds>Reminder Alerts

The phone will always follow the setting in Sounds, not the one in Notifications.  The same applies to calendar alerts.  Sigh.