For reasons that are unclear to anyone outside of Facebook, the iOS and Android apps make a ton of noise. There's a sound when you refresh your feed, when someone starts typing or when you get a new alert. And frankly, the sounds are annoying.
Thankfully, you can disable the in-app sounds in a matter of seconds.
Disable on iOS
On an iOS device, tap on the button with three lines in the bottom right corner of the app. Next, scroll down and tap on the dropdown labeled Settings & Privacy then select Settings. Scroll through the list of Settings until you find the Sounds option and tap it. Finally, slide the switch next to In-App Sound to the Off position.
Disable on Android
The process on an Android device is similar to iOS. Open Facebook and tap on the three-lined icon along the top of the screen. Scroll down until you find Settings & Privacy, select it, then tap on Settings. Next select Media and Contacts and then toggle Sounds in the app to the Off position.
Originally published on Feb. 13, 2015.
Update, Nov. 15, 2018: Added new screenshots and instructions.