Sean Ludwig, with VentureBeat, noticed a new feature that Facebook is testing because his profile is part of the small test group. The new "feature" is that Facebook plays a sound whenever you get a new notification. I know what you're thinking: they already do this--that's only on the mobile app version.
This trial run pings you whenever you receive a message or notification. Ludwig notes that it is a bit of a gamble leaving your Facebook open and your speakers on: "Will I hear a lot of pings for that new picture I posted, or will I hear nothing?" These sounds certainly have their place and some users may even like them.
Myself, since I receive a fair amount of notifications, none of which are life-threatening, would not like these constant pings. Luckily, they're simple to turn off. The same way you change around e-mail notifications and the like, you can turn off the sound that plays when you receive notifications: