If you're finding that you're able to get picture but not sound, it's likely to be one of two things:
Most commonly, you have a loose SCART plug - the audio pins on a SCART plug are at one end of the SCART connector. If the plug isn't in straight, the audio gets disconnected, but the video doesn't. Try reconnecting the SCART plug (at both ends) ensuring that the connectors are in and straight. It's also worth trying a different SCART cable in case the pins are damaged.
The Freeview box could be in "mute". Many boxes have their own audio control separate from the TV. Check that the box isn't in mute, the volume isn't down, and that there's nothing wrong with any of the volume/audio settings via the box's onscreen menu. The manual may be of help here.