Either the TV or the Laptop might have to have a setting enabled to allow sound to go over the HDMI. On my computer it's accessed by right clicking the speaker icon in the notification tray and selecting "playback devices". If sound over HDMI is allowed, then a device such as a S/PDIF device will be an option.
The TV might also need to be told to expect sound over HDMI, but usually if sound is present the TV will work it out for itself!
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