How do I stop my Mac from ringing when I get a phone call on my iPhone?

Yosemite has a new feature where my Mac rings when I get a phone call. Annoyingly, it continues to ring for 30 seconds or so, even after I've answered the iPhone.

How can I turn this sound off? I'd like to continue to have this feature available on my Mac: I'd like to be able to start calls from my Mac and I'd like to see the little alert that says "Accept / Decline" when the calls come in, but I want to turn off the ring sound on the Mac so I can answer my iPhone without having the other person hear endless ringing in the background.

Easy Peasy.

System Preferences > Notifications > FaceTime > uncheck 'Play sound for notifications'

Alternatively, you could just turn on 'Do Not Disturb' from the Today/Notifications widget easily when you need it by clicking on the notifications button in the menu bar and scrolling up the 'Do Not Disturb' setting will scroll into view

Launch /Applications/FaceTime.app

Click on FaceTime > Preferences

Uncheck "iPhone Cellular Calls"