Whispers of Terror

The Doctor and Peri find themselves in the Museum of Aural Antiquities, where every sound is stored for posterity - from the speeches of Visteen Krane to security service wire taps and interrogation tapes. But they also find an intruder, mysteriously changed recordings, and a dead body.
Before long the Doctor realises that there is more going on than a simple break-in or murder. How can he defeat a creature that is made of pure sound?