It seems that Facebook for Android is adding a new UI supporting a new feature related to ringtones. The new UI will allow to pick sound files from the device and set as ringtones.
Check the image in the tweet below that shows the new UI for setting sound files from the device as ringtones in Messenger.
Facebook Messenger for Android is testing a new UI for picking Ringtone and Notification— Jane Manchun Wong (@wongmjane) August 28, 2019
This new UI allows adding sound files from device pic.twitter.com/TtVxpN3Mev
This has been revealed by Jane on Twitter. She is known to reveal new features and changes under testing for popular apps. It also seems that this new UI will also allow picking notifications, though Jane has not dwelled on that in her tweet.