Sound alert for other people's messages
For people connected as moderator or above, it plays a sound when someone else's message arrives. It isn't a mobile push notification.
Good to know
It stays quiet for about 1 second right after joining. After that, messages from other nicknames make a sound even while you're looking at the page.
If you hear nothing, check whether the browser allows sound, whether the tab is muted, and your device volume.