Notifications are not working since updating to Monterey

Hello N_nicoll,

Thanks for reaching out to Apple Support Communities. We see you have some questions about notifications on your Mac, and we'd like to help. The following link may be able to help with how to Change Notifications preferences on Mac.

Regards.

If you use an external monitor or anything like that, this solution worked for me...

I had that issue too; before updating, I would get a sound indicating a new message for Slack; and after updating I no longer got that sound. After trying numerous things suggested online, I finally figured it out for myself.

System Preferences > Notifications and Focus

Make sure the bottom box is checked "When mirroring or sharing the display". Once I had that checked, it immediately fixed my issue.