This is only about the Welcome video on your Home page (usually Home). It is not a video call with family, and it is not about My Circle.
Many browsers start that video with sound off. That is normal. You did nothing wrong.
How to hear it
Stay on Home.
Tap or click the Welcome video itself.
Sound should turn on. If captions appear, you can read along too.
If there is still no sound after you tap the video, check that your device volume is up and that the browser tab is not muted. Then reply in chat and ask for a human. Say you are on the Home Welcome video and still cannot hear it.
What this is not
Do not follow help for Circle video calls, mute on a family call, or Family & Friends → Video Calls. Those are a different feature.