Facebook Messenger isn’t making a sound when I send a message

If you don't see notifications for a specific app

To get notifications, connect to a Wi-Fi or cellular network. If you still don't get notifications for a specific app, try these steps:

1. Make sure that the app supports notifications. Go to Settings > Notifications, select the app, and make sure that Allow Notifications is on.
2. If you have notifications turned on for an app but you're not receiving alerts, you might not have Banners selected. Go to Settings > Notifications, select the app, then select Banners.
3. Make sure that you’re signed in to your Apple ID. 
4. Make sure that Do Not Disturb is off. 
5. If you’ve recently installed an app or restored it from a backup, open the app to start getting notifications.

Change notification settings on iPhone

1. Go to Settings > Notifications.
2. To schedule a notifications summary, tap Scheduled Summary, then turn on Scheduled Summary. ... 
3. To choose when you want most notification previews to appear, tap Show Previews, select an option—Always, When Unlocked, or Never—then tap.

More items...

Change notification settings on iPhone - Apple Support (IN)

Facebook is not made by Apple, so we cannot help you here. You'll need to contact Facebook.