Calendar notifications stopped working after Sequoia update

That gave me the hint I needed after, for my wife, upgrading from a Mac mini with High Sierra to a new iMac. I had exported (from HS) and imported all events into Sequoia. But no notifications for either old repeating events or for new events.

Rather than removing the Audio Actions from each event, I copied all the system sound files from High Sierra to ~/Library/Sounds. Now Reminders notifications work for all events.

Just to add another reply, I have now got mine working. It was one of these things - sorry I did them all at the same time, can't tell you which exact thing solved it.

I deleted as many old events as I could using the search function

I exported the calendar to ics, deleted my calendar

I opened the ics, and removed any fields that had references to system sounds that no longer exist in Sequoia

ie this part of an event

ACTION:AUDIO

ATTACH;VALUE=URI:Basso

Each event starts with BEGIN:VEVENT and ends with END:VEVENT, so i was able to find some more old calendar events to delete

I deactivated the new "Apple Intelligence" that came with the update

I imported the ics into a new calendar.

My alerts are (for the moment) working again

Same, and new events only work if you set them more than about 10 minutes into the future. Most old events since before the update have stopped working completely