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Messenger allows users to choose from a variety of “text tone” notification sounds in each of their conversations. The “text tone” is the notification sound that plays when a user receives a new message in a conversation.
Our guide will show you how to change the text tone in a chat in the Messenger mobile application.
Note: These screenshots were captured in the Messenger app on iOS.
Step 1: Open the Messenger conversation you want to customize and tap the conversation name at the top of the screen.

Step 2: Under the “More actions” section, tap “Notifications & sounds.”

Step 3: Tap “Text tone.”

Step 4: Tap one of the sound options to select it. Note: You can tap each tone option to preview it before making your final selection.

https://www.adweek.com/social-marketing/messenger-how-to-change-the-text-tone-for-a-chat/