Francis McCarty used a high-frequency spark for his audio transmissions, which wasn't too practical for high quality sound. But this young genius never got the chance to perfect his ideas, for he died fewer than three years later. For more information about Francis McCarty's short life, see John F. Schneider's Francis McCarty, Early San Francisco Radio Experimenter.
Saint George (Utah) Dixie Advocate, January 7, 1903, page 5:
15 Year Old Boy is Said to Have Found How to do it.
A San Francisco dispatch says: Has 15-year-old Francis McCarty solved the problem of wireless telephonic communication? The youngster is confident that he has. His friends have absolute faith in his invention, and the enthusiastic youngster, who has already succeeded in transmitting his wireless messages four miles, will, in the course of the next few weeks transmit wireless messages to some other point yet to be selected. Time alone will tell whether or not this boy has succeeded with his crude appliances in accomplishing that which other inventors have expended hundreds of thousands of dollars in and met only with failure, or, at best, only partial success.
Francis McCarty, through a series of experiments, has discovered an apparatus which will take up the variations of the voice and transmit them wirelessly to great distances. In other words, this fellow, still in the blush of adolescence, has invented a wireless telephone which will carry messages with perfect distinctness and clarity and a distinctness and clarity that vibrating wires from the nature of things can not possibly introduce. His apparatus is simple and uncumbersome, connections being instantaneously made.
He has based his principle upon the Hertzian-wave theory, the theory Marconi and other inventors of wireless telegraphy have based all their experiments upon. His full intent in the beginning of his long distance experiment was to merely establish a system of wireless telegraphing, employing his own workout principle of variating the spark from the ordinary "jump-spark" coil by a transmitter when the transmitter is variated by a telegraph key and a coherer, another device of his own, which will take up the variation of the spark.
In purely an accidental manner he found that his fertile brain had managed to get the two windings of the induction coil employed into such sympathy that the transmitter, when variated by his voice, in turn variated the spark, his discovery was made when he was making his four mile test from his home at 439 Gough street to Twin peaks.
He had left a telephone transmitter attached to the receiving station on the bill which he had left in charge of his father and brother, and he at the receiving station, established on the porch of his home, heard to his own surprise distinctly words his father was addressing to his brother four miles away.