Aw sorry, I did not understand it !
Well, running Sierra or Yosemite will not be relevant in this situation.
And as Dialabrain said, there's no Terminal command to do such a thing. Sounds are hidden in deep locations.
This works because the Calendar and Reminders both rely the system sound "Basso" as their default alert sound.
That sound is located in /System/Library/Sounds and is called Basso.aiff
Here's an unofficial solution to fix the issue:
1. Delete Basso.aiff
2. Find a better alert sound
3. Change the name of the sound to, "Basso" and convert it to AIFF so it should now be called, "Basso.aiff"
4. Move the file to /System/Library/Sounds
5. Restart the computer
6. Now your default alarm for Calendar and Reminders has a great sound.
Hope this will help,
Cheers !