I guess IGAF😂
I've had this problem the last week, samsung rep rest all app references via the tutor app, worked for a while. Performed factory reset myself which worked for a while, samsung agreed to replace the hadset.
Here's what I did to fix it, by fix I mean i'm 24hrs issue free with notifications alerts working as expected, previous best was 2hrs max before notifications all muted and vibrated only.
power phone off and boot to bootloader screen
Select repair apps, run it
Select wipe cache partition, run it
I then selected mount/system and ran it, wiped the cache partition again for good measure.
Then i selected reboot system now.
After which phone restarts and the small screen for optimising apps pops up where it run through all the apps, same as what you see after a software upgrade.
I found getting in to the bootloader a bit or a nightmare, this is the only surefire way I got into it - https://www.youtube.com/watch?v=cd_caLuD-eY
What I did might be over kill, you may only need to select repair apps, or just wipe cache partition.
After I had some wifi issues but that was yesterday, today all seems fine ( putting it down to router issues, mesh system here) For now I am going to hold off sending the phone back for a replacement.
Hope you get sorted.
I am having this same issue. Tried booting to the bootloader wiped cache factory reset mounted system and repaired apps. After all that notification sounds still didn't work. I set up my phone fresh and checked and they did not. Also seems that it's notification and system sounds. What's strange is if you go into volume and adjust the ring tone it will start playing the ring tone. If you touch the notification volume slider while the ringtone is playing you will hear one chime for the notification but if you move it again when the ring tone is not playing it does not make a noise. I can only guess this is some sort of bug in the last update
I had the same issue and just realised that actually the notification sound volume had dropped to zero.
Don't know if its as simple as this but doing the below solved it for me on S21: