The Music I was the first software for sound synthesis, developed by Max Mathews at Bell Laboratories in the fifties.
Background history – With the Music I also began the history of the Music N family of software for computer music. It was completed in 1957 at Bell Labs in Murray Hill, New York. The working group, in addition to Max Mathews, also involved John Pierce and Newman Guttman. Rarely encountered called Music 1.
Technical aspects – The software was developed to run on IBM 704, of which he was using the assembler. The use of machine language, before a choice was a need. In fact, because of the poor performance of the computer age than today, the assembler guarantee greater stability and a higher computational speed. It should be noted that even today, such as Csound, we find expressions such as GO TO, AND, NOT, characteristics of the machine language. The Music I was a simple monophonic instrument, able to generate a single sound wave (triangular) with no attack and extinction transient. It was only possible to set the parameters of amplitude, frequency and duration of each sound. In essence it was not possible no control over the sound. The output was stored on a magnetic tape, then subjected to the process of conversion by a DAC. At the end of the process, so you could hear the sound result. This, moreover, made it possible to also generate sounds very long. Bell Laboratories, in those years, were the only ones in the United States, to have a DAC (in that case, a 12-Bit valve technology converter, developed by EPSCO). Bell Laboratories, in essence, were the only ones to do a job like that, Mathews writes that :
In fact, we are the only ones in the world at the time who had the right kind of a digital-to-analog converter hooked up to a digital tape transport that would play a computer tape. So we had a monopoly, if you will, on this process
Added to this is that few had the availability of an IBM 704 machine, very handsome in those years, and few had access to a magnetic recorder such as that of IBM in New York, in which to store the output of the programming. The question had made complicated by the fact that the IBM 704 was based at IBM’s headquarters in Manhattan, where he was also a tape recorder, while the DAC was stationed at the headquarters of Bell Labs, for an hour and a half drive from Manhattan. One approach now hard to believe; quite normal in those years.
The first computer music – On May 17, 1957 Mathews and his colleagues generated a synthesized melody of 17 seconds through the computer and Music I. It was a short composition, so called by its author Newman Guttman, entitled The Silver Scale. Experimental work, doubt aesthetic value, but whose historical importance is certainly interesting. In the same year, yet Mathews and Guttman, realized a new work: Pitch Variations. Lasting about a minute, this additional experiment aimed to investigate some issues related to the perception of pitch. These works, along with many others, were collected in an anthology entitled Music From Mathematics (1962) edited by Bell Labs. The anthology gathers works made with the Music I, Music II and Music III. Not satisfied with the aesthetic results, Mathews carried on their work developing the Music II.
For this topic I’ve read.
 AA. VV. The Canonical Csound Reference Manual: Version 5.08, pp. 211 – 213.  Curtis Roads, Interview with Max Mathews, Computer Music Journal, Vol. 4 , 1980.