The hypnosound was a soundwave that affected the fear centres of the mind. It seemed only to affect the weak-minded, such as Ogrons. Once people chose to believe it was an illusion, they were unaffected by the soundwaves. The strong-minded could resist it.
The Master created a device that sent out hypnosound waves to provoke war between Earth and Draconia in the 26th century on behalf of the Daleks. When Jo heard this sound, she saw a Drashig, a Mutt and a Sea Devil. The humans of Earth saw Draconians attacking their ships and the Draconians saw humans, when in fact it was actually Ogrons. When the Third Doctor and his friends were imprisoned on the Ogron home planet, the Doctor used this device on an Ogron to let them escape. The Ogron saw the golden Dalek Supreme and immediately let the Doctor and his friends out of their cells. (TV: Frontier in Space)