@H747H: Is the issue occurring under certain conditions e.g. while the phone is idle, or is it persistent?
If you head to Settings > Notifications > App notifications, are the affected apps toggled on here? If so, try heading to Settings > Apps > Select an affected app > Notifications > Allow sound and vibration (On) > Notification categories > Ensure all relevant categories are toggled on, then tap on a category and select 'Alert', followed by Sound > Choose any option except 'Silent'.
