Unfortunately Android doesn't have default integrated functionality to notify user periodically about missing calls/messages/other notifications via sound (only LED is blinking). This opensource tool allows to monitor notifications from any applications which are important for You (including missed calls, messengers, SMS, e-mails, etc) and perform sound reminder periodically 

Just launch app, allow it to receive notification events, select reminder interval and important applications. That's it. If there are any unhandled notifications from thet selected apps in the statusbar, app will notify You periodically with the notification sound you choose.

IMPORTANT: if app doesn't work please try to find solution for your phone here https://dontkillmyapp.com

The battery is used only if there are missed notifications which requires periodical reminder. App wakes phone to perform sound notification and then sleep again.

The app uses minimum set of required permissions.

Android 4.0-4.2 support is experimental such as these OS versions doesn't have a 100% working method to monitor notification removal.

XIAOMI users should enable application autostart explicitly in the security settings
Huawei users should disable battery automanagement for the Missed Notifications Reminder app: in the  Battery > App Launch -> Manage All Automatically - - >  uncheck MNR app

Source code is available at github: https://github.com/httpdispatch/MissedNotificationsReminder

Big thanks to all contributors:
Sergiy Belozorov (https://github.com/rryk) for limit reminder repeats and dismiss notification features
naofum (https://github.com/naofum) for Japanese translation
Gerasim Panteleev for bulgarian translation
Heimen Stoffels (https://github.com/Vistaus) for Dutch translation
Olaf Kuba for design updates

Periodical sound reminder for missed/unread notifications (Calls/SMS/Etc).