For reasons unclear to anyone outside of Facebook, iOS and Android apps give tons of sound. There is a sound when you update your feed, when someone starts typing or when you get a new warning. And frankly, the sounds are annoying.
Thank you, you can disable in-app audio in seconds.
Disable on iOS
On an iOS device, press the three-line button in the lower right corner of the app. Then roll down and tap the drop down menu with Settings and Privacy and then select Settings. Scroll through the list of settings until you find the Sound 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 the three-digit icon at the top of the screen. Scroll until you find Settings and Privacy select it, and then press Settings . Then select Media and Contacts and then select Sound in the app to Off the position.
Originally published February 13, 2015.
Update, November 15, 2018: Added new screen shots and instructions.