If you are browsing m.facebook.com on your Android phone using browsers which support running user scripts, such as Yandex browser or Firefox browser, and you want to hear a notification sound every time a message arrives, then my script is very suitable for your need.
Version 2.0: Add vibration.
Version 2.1: Vibration time change from 300ms to 500ms.
Version 2.3: Adapt to new version of facebook.
Version 3.0 Complete rework for new facebook.