How can I change the default alerts of Calendar (iCal)?
As far as I know, these are the only two ways to change Calendar alert sounds. In both cases, unfortunately, they are universal changes—there is no way to override them for individual events.

Disable all Calendar alert sounds

Go to System Preferences > Notifications > Calendar and disable "Play sound for notifications."



Change all Calendar alert sounds (pre-El Capitan)

We'll be digging around in System a bit, so be prepared to enter your password a couple times. I'm on El Cap now (my only Mac), and this isn't working for me, but I know it worked in the past. My guess is System Integrity Protection. This should probably work if you're on an earlier version or have SIP disabled; let me know if it doesn't.


In the Finder, go to /System/Library/Sounds
Change the name of Basso.aiff to something like Basso-original.aiff
Add a new audio file of your choice named Basso.aiff