Although the only audio that operators heard when receiving along Morse telegraph lines were the clicks of the receiver, in 1860 there was already speculation about transmitting more complex sounds. However, it wouldn't be until the middle of 1870s that the telephone would be perfected to the point that it would come into regular use.
History, Theory and Practice of the Electric Telegraph, George B. Prescott, 1860, pages 334-336:
MUSIC BY TELEGRAPH.
It is an amusing fact, that music has actually been transmitted by the Morse telegraph, by means of its rhythm; in fact, it is of very frequent occurrence upon all lines. The following is related by Mr. Jones, who was an ear-witness of the experiment in New York : --
"We were in the Hanover Street office when there was a pause in business operations. Mr. Porter, of the Boston office, asked what tune we would have. We replied, 'Yankee Doodle;' and to our surprise he immediately complied with our request. The instrument commenced drumming the notes of the tune as perfectly and distinctly as a skilful drummer could have made them at the head of a regiment; and many will be astonished to hear that Yankee Doodle can travel by lightning. We then asked for 'Hail Columbia!' when the notes of that national air were distinctly beat off. We then asked for 'Auld Lang Sync,' which was given, and 'Old Dan Tucker,' when Mr. Porter also sent that tune, and, if possible, in a more perfect manner than the others. So perfectly and distinctly were the sounds of the tunes transmitted, that good instrumental performers could have had no difficulty in keeping time with the instruments at this end of the wires."
That a pianist in Boston should execute a fantasia at New York, Philadelphia, Washington, and New Orleans at the same moment, and with the same spirit, expression, and precision as if the instruments, at these distant places, were under his fingers, is not only within the limits of practicability, but really presents no other difficulty than may arise from the expense of the performances. From what has just been stated, it is clear that the time of music has been already transmitted, and the production of the sounds does not offer any more difficulty than the printing of the letters of a despatch.
It is well known that the pitch of any musical note is the consequence of the rate of vibration of the string by which it is produced, and that the more rapid the vibration the higher the note will be in the musical scale, and the slower the vibration the lower it will be. Thus the string of a piano-forte which produces the base note vibrates 132 times in a second; that which produces the note vibrates 66 times in a second; and that which produces the note vibrates 264 times in a second.
On a seven-octave piano-forte, the highest note in the treble is three octaves above , and the lowest note in the base is four octaves below it. The number of complete vibrations corresponding to the former must be 3,520 per second; and the number of vibrations corresponding to the latter is 27½.
By means of very simple expedients, the current may be interrupted hundreds or even thousands of times in a second, being fully re-established in the intervals. If the pulsations of the current be produced at the rate of a thousand per second, the alternate presence and absence of the magnetic virtue in the soft iron will equally be produced at the rate of a thousand per second. Nor are these effects in any way modified by the distance of the place of interruption of the current from the magnet. Thus, pulsations of the current may be produced by an operator in Boston, and the simultaneous pulsations of the magnetism may take place in New Orleans, provided only that the two places are connected by a continuous series of conducting-wires.
When it is stated that the vibrations imparted by the pulsations of the current to levers have produced musical notes nearly two octaves higher than the highest note on a seven-octave piano, tuned to concert pitch, it may be conceived in how rapid a manner the transmission and suspension of the electric current, the acquisition and loss of magnetism in the soft-iron rods, and the consequent oscillation of the lever upon which these rods act, take place. The string which produces the highest note, on such a piano, vibrates 3,520 times per second. A string which would produce a note an octave higher would vibrate 7,040 times per second, and one which would produce a note two octaves higher would vibrate 14,080 times per second.
It may, therefore, be stated, that by the marvellously subtile action of the electric current, the motion of a pendulum is produced, by which a single second of time is divided into from twelve to fourteen thousand equal parts.
The adaptation of this power to the production of music upon telegraphic piano-fortes at any distance which may be desired, is a matter of the utmost simplicity, capable of being successfully carried into practice by any one who has the money and taste for the experiment.