Classes are muted by default because many web browsers do not allow autoplaying video with sound (ads would get really annoying otherwise!). If your browser has muted the live stream automatically you'll need to unmute your instructor manually. After a few classes your browser should learn that you want to allow audio by default for our website, and unmute automatically.
To enable sound, click the unmute icon (🔇) on your instructor's video:
Once you click the icon you should be able to hear your instructor.
If you've done that and still do not have sound, please check the volume level on your device and that your audio is not muted. Also check that you don't have headphones plugged in or connected to your device via bluetooth. It sounds silly, but it's easy to forget in the moment, and has happened before!