How do I change the default sound for Reminders in El Capitan? It is very important as I use it for medication reminders. I would like to use one of the sounds (.aif) that I already have stored on my computer. Changing default sounds in Calendar would be helpful to know as well. 

I've followed the steps at cultofmac previously, but tried again. My ~/Library/Sounds folder is empty. I added a newly created aiff and named that sound of my choice as basso.aiff and placed it in the empty folder. Followed the terminal instructions. But, I get no alert sound at all from Reminders. 

Strangely, alert sounds (ie notification of deleting a song from iTunes, as well as certain alerts in other apps use that aiff too.) I could careless about what sound I hear in any app other than Reminders. Suggestions?
You were on the right lines, but the wrong location…

System sounds are in /System/Library/Sounds/ it's only user sounds in ~/Library/Sounds
Replace the Basso.aiff in there with your custom version [bearing in mind this will replace Basso in any other system alert that uses it too]  

It will be the same for Calendar alerts - except there you can define which system sound to use, so if you did need a separate custom sound in there you could apply the same logic to one of the other system sounds in the Library folder.



Note that in El Capitan you would first have to disable System Integrity Protection before you can modify the Sounds folder…


  
  Reboot your Mac into Recovery Mode by restarting your computer and holding down Command+R until the Apple logo appears on your screen.
  Click Utilities > Terminal.
  In the Terminal window, type in csrutil disable and press Enter.
  Restart your Mac.
  
  
  Revert the same way using csrutil enable