This isn't rolling out for everyone just yet. Gee, I guess I'm just lucky. When it comes your way, trust me, you'll be wanting to disable it ASAP. Thankfully that's not difficult to do:
Go to 'Account Settings > Notifications'.
Click 'View' on the tab that says 'On Facebook'...
...then unclick the box that says 'Play sound when each new notification is received', press 'Save Changes', and relax.
NB: It might not go off at first, if (like me) you have multiple tabs or accounts open. You will need to do this for any alts. if you have any (I know I do, for old legacy billing with certain clients).