This article explains how to change the sound on Android when you get a Facebook Messenger notification.
Steps
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Step 1. Open Messenger
It is located in the app drawer and the icon looks like a blue speech bubble containing a white lightning bolt.
If you are not already logged in to Facebook, enter your details to do so
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Step 2. Tap the profile settings icon
It depicts a gray circle containing a white human silhouette and is located in the upper right.
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Step 3. Tap Notifications & Sounds
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Step 4. Swipe the "Notifications & Sounds" button to activate it
If the key is already blank (therefore active), you can skip this step.
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Step 5. Swipe the "Sound" button to activate it
If it is already blue (therefore active), you can skip this step.
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Step 6. Tap Notification Sound
It is located just below the "Sound" button.
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Step 7. Select a sound
Tapping the sounds listed in the list will hear a preview.
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Step 8. Tap OK to save your selection
From now on when you receive notifications on Facebook Messenger the Android device will emit the chosen sound.