Some types of signals are based on making a noise that can be heard a long way off. The air raid sirens of the Second World War were a way of signalling by sound - using a rising and falling siren to tell people that enemy bombers were on the way and a steady note to give the 'All Clear'.
Other types of sound signals include bells, horns, guns, or the tom-tom drums used by tribes in Africa, New Guinea and South America to pass messages.
The trouble with sound signals is that the range is so uncertain. In clear country and with no other background noise, a gun or siren could be heard several miles away. But in a noisy city, with trees or buildings in the way, the sound might not be heard at all.