Small script for a request https://greasyfork.org/uk/forum/discussion/6631/change-facebook-notification-sound-convert-chrome-extension-to-userscript
You can find settings option in Greasemonkey's "User Script Commands..."

UI is quite simple: Enable, Sound File URL and Notification Check Interval (in seconds).

Small script for a request https://greasyfork.org/uk/forum/discussion/6631/change-facebook-notification-sound-convert-chrome-extension-to-userscript