How to re-enable sounds for calendar notifications?

I have my work Exchange account syncing with the default Mac OS Calendar app. Until recently, this worked very well and I would get notifications (with sound) for forthcoming events.

For the last week or so, the notification sound has vanished. I still get pop-up notifications at the right time, but no sound.

My exchange account presents three calendars: "Calendar", "Birthdays" and "United Kingdom holidays". Interestingly, the holidays calendar showed an alert today and there was a sound! So the problem is limited to my 'Calendar' Exchange calendar.

Here's what I've tried so far:

• I've looked in Notifications in System Preferences and ensured Calendar has "Play sound for notifications" enabled. I've also tried disabling and re-enabling this, to no avail.
• I've looked in Sound in System Preferences and ensured alert volume is set appropriately. I've also got "Play sound effects through" set to MacBook Speakers.
• Based on an answer below, I've tried removing the Exchange account from my Mac and re-adding it. No change.
• I'm running Catalina 10.15.13 on a MacBook Pro (13-inch, 2019, Four Thunderbolt 3 ports).

Here are the alert settings within Calendar:

Any suggestions for how to resolve this?

I've been dealing with Exchange/mail.app on iOS issues at my company for years and the only reliable way to deal with this problem is one of two ways.

1. Delete the Exchange account from your iOS device and re-add it. Since everything on Exchange is kept on the server you will lose nothing but the time it takes to do this.
2. Move your Exchange-based email account(s) to (the free) MS Outlook app for iOS.

Both of these seem like unfortunate choices, but both work. The Outlook email client is actually pretty good and (for me) allows me to keep my work and personal emails completely separate.

The issue with Exchange and mail.app not syncing up properly from time to time (and other, ahem..., issues) is an ongoing issue that (I have heard but cant find the source to cite) MS Blames on Apple and vice-versa. So there is no actual fix for the problem. Deleting the Exchange account for mail.app forces it to re-sync everything and (9 times out of 10) will fix the issue. Until it happens again, which is one of the reasons I migrated my work email to the outlook.app from MS.

If someone has discovered a permanent fix for this I would love to hear about it as it would save me that in-person interaction with (yet another) user and a lot of wasted time fiddling with it. So far it still exists on iOS 13.

System Preferences -> Notifications -> Calendar