By default Safari 11 prevents any stream comprising video and sound from auto playing unless the site has been previously authorized.

This might freeze the streams of the chatrooms (you would see just the firt frame) if you are coming onr this site for the first time after updating to Safari 11

To solve the problem you need to open Safari's Preferences, tab "Web Sites" select "Auto Play" in the left column then authorize this site

However we are studying this issue to see if we can solve it from our end