I have a Samsung a20e may also apply to other models, had no wakeup alarm or notification sounds, went into safe mode (hit off key, hold finger on power off icon until safe mode icon appears) and all notifications worked, I installed McGraw Hill Language lab app last week (also did an OS update last week also) so thought it may have been an updated OS version problem, I uninstalled the Language Lab app and all Alarm/Notifications working again. Of course this is very frustrating as I spent several hours troubleshooting, powering down, going into recovery menu, clearing cache etc. but must be frustrating for Samsung techs also as it is easy to blame the phone (might be Hardware Problem) or the OS Version, but if things work in Safe Mode I would say it would be a bad app that was installed that is causing the problem. Just keep track of last apps installed etc.
Looks like I spoke to soon, the phone has once again gone silent on wakeup alarms and Notifications, going into safe mode and then uninstalling the last app I installed seemed to be only a temporary fix. I hope that there will be a fix for this problem, I have now had to go back to my old phone for Alarms and Notifications as I can not trust the a20e as I use my phone for may reminders and wakeup alarms etc.