There is no option to disable notification sound directly in the app. You can only turn notifications on and off.

App info settings directly in Android provide “Notifications” setting.

At first glance, it seems you can only enable or disable Default or Calls notifications with checkboxes, and to Allow notifications dot.

You have been deceived!
Both Default and Calls are clickable!
Click on Default and configure its Behavior – set it to “No sound” or “Show silently and minimize” to disable notification sounds.

Source: https://www.reddit.com/r/GooglePixel/comments/9bvo91/cant_disable_fb_messenger_sound_on_pixel/e5d4lsm/
…aaaaand the notification nightmare is over!
Feeling grateful? 😄 Click the button below:
[kofi]
9 responses to “How to Disable Annoying Facebook Messenger Notification Sound on Android”
Thank you, this was very helpful!! I finally got my messenger to be quiet! Facebook certainly did not make it easy with the hidden menu.
Thank you. First helpful post on how to disable these.
Thanks, finally figure out how to make Messager notificatin silent
How do i disable the in-messenger notification that i have notifications turned off?
Mate you’ve legitimately made my life better with this post!
I yelled out I’ve been deceived when I read this
Haha, awesome!
YOU ARE AWESOME!!!!
You are truly heroic, this has been annoying me no end, and this was the only article that I found to address the problem correctly!
FINALLY someone solves the problem instead of copy-pasting that old “click the turn off sound button” answer! It’s strange that Facebook never fixes the problem from the start, since “no sound” should really mean NO sound.
But a big thanks! You saved my sanity!