For reasons unclear to anyone outside of Facebook, the iOS and Android apps cause a lot of noise. There will be a sound when you update your feed, when someone starts typing, or when you receive a new notification. And frankly, the sounds are annoying.
Fortunately, you can disable in-app sounds in seconds.
Disable the iOS feature
On an iOS device, tap the three-row button in the lower-right corner of the app. Scroll down and touch the Settings & Privacy drop-down menu . Then select Settings. Scroll down the list of settings until you find Sounds and tap it. Finally, move the switch next to In-App Sound to the position Off .
Disable on Android
The process on an Android device is similar to iOS. Open Facebook and tap the three-line icon at the top of the screen. Scroll down until you find Settings & Privacy select it and tap Settings . Next, select Media and Contacts and then switch .
Originally released on February 13, 2015.
Update dated November 15, 2018: New screenshots and instructions added.