Unfortunately Android OS 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 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 the selected apps in the status bar, app will notify You periodically with the notification sound you choose. 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. Video at https://youtu.be/5xvi7q1_pmo