Facebook is trying out new auto-play videos in the News Feed that include sound, a change few people have asked for. A small number of users have reported encountering videos that automatically play with the sound enabled unless your phone is on silent mode or audio is disabled. Other users have seen videos that include an icon for toggling the sound on and off.
It appears this test is only live for a subset of Australian users and there’s no indication Facebook is expanding the test. Those who are affected by this test can turn off sounds in the Settings area or silence their phone.