Open the Messenger app
Tap your profile picture in the top left
Tap Notifications & sounds
Tap Notification sound
Select the sound you want
Go back to save the change
If needed, check your phone’s system notification settings for Messenger
On iPhone, go to Settings > Notifications > Messenger > Sounds
On Android, go to Settings > Apps > Messenger > Notifications > Notification categories > Sound
Make sure Messenger notifications are enabled in both the app and your device settings