Presuming you mean facebook videos, have you tried hovering your mouse pointer on the video which will reveal the sound icon. It may be muted and this happens to me but don't ask me why, I don't really do fb that much.
What Browser are you using? Edge? Firefox? Chrome? Open up FB and play a FB video then right click the Volume icon in your taskbar and Open Volume Mixer. You should see your browser with it's own Volume Control, it might be muted, or too low
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