Classes are muted by default the first time, because web browsers do not allow autoplaying video with sound (ads would get really annoying otherwise!). To hear your instructor, you'll need to unmute them the first few times you take class with us. 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 that, you should be able to hear your instructor. If you've done that and still do not have sound, please check the volume on your device, and 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!