If you have installed the Facebook App on your Android smartphone like Samsung Galaxy S9, Huawei P20 Pro, Sony Xperia XZ 2 etc., you might also use so called reactions under contributions from friends or channels.
These are smileys, hearts, like buttons etc. Every time you send such a reaction under a post, you will hear a sound. If this sound, which comes from the Facebook app, disturbs you, then you can deactivate it as follows:
1. Open the Facebook app on your Android mobile phone.
2. Tap on the 3 bars in the top right corner.
3. Tap on App settings and search for "Sounds for Reaction" in the "Media for Reaction" area.
4. Deactivate the option "Sounds in the App"
If you now make a reaction, then no more sounds from the Facebook app are audible.