When each new notification is received a sound is played by default in Facebook . So as to turn off Facebook notification sound just follow the steps below.
1 Facebook Login
2 Then on upper-right corner click the gear menu of any page on Facebook and click on “account setting. ”
3 Now click “notifications” in the left hand side of the page.
4 Find the “how you get notifications” section.
5 Click “view” next to “on Facebook.”
6 Unchecked the box next to “play a sound when each new notification is received.”
7 Click “save changes.”