Sound won't play on iPads and iPhones

There is an issue with Safari on iOS that prevents sound from playing in Dance Party and other labs when the device is in silent or "Do Not Disturb" mode.

For iPads, this is controlled through the Control Center (more information).

For iPhones, this is controlled using the ring/silent switch (more information).

Make sure the device is not in "Do Not Disturb" or otherwise silenced.