He began his conducting studies under the legendary conductors Nikolai Malko and Aleksandr Gauk. In 1934, Musin became assistant to Fritz Stiedry with the St. Petersburg Philharmonic Orchestra. Later, he was sent by the Soviet government to lead The State Belorussian Orchestra. Because Musin never joined the Communist Party, his conducting career was curtailed by the Soviet government; he then channelled his creative output into pedagogy, creating a school of conducting, still referred to as "Leningrad conducting school." During the war years (1941-45), he was in Tashkent, Uzbekistan, where most of Russian intellectual forces were kept safe; he continued conducting and teaching. On the anniversary of the Nazi invasion (June 22, 1942), he conducted a performance of Shostakovitch's Seventh Symphony; this was the second performance of the work, after it had been premiered by Samuil Samosud in Kuybishev.
In 1932, Musin was invited to teach conducting at St. Petersburg Conservatory (then Leningrad Conservatory). He developed a comprehensive theoretical system, truly a science of the conducting gesture, mastering of which allowed the student to communicate with orchestras with the hands only, with minimum of verbal instructions, down to the minutest detail. No one hitherto had tried to formulate a science of conducting gesture in such detail and clarity as Musin did. Apparently, his own early years as a student of conducting had prompted him to study the intricacies of manual technique; the explanation is that when in 1926 Musin tried to enter Malko's conducting class at The Leningrad Conservatory, he was denied entrance for the reason of his weak manual technique. He pleaded Malko to be accepted on a provisional basis, and over the years in fact became the supreme authority on manual technique, having developed a system, which he expounded in his book "The Technique of Conducting." This fundamental work is still available only in Russian; since the day of printing (1967), it has been a rarity even in Russia. Musin formulated the main principal of his method in these words: "A conductor must make music visible to his musicians with his hands. There are two components to conducting, expressiveness and exactness. These two components are in a dialectical opposition with each other; in fact, they cancel each other out. A conductor must find the way to bring the two together."
His pedagogical career spanned 60 years of teaching; his pedagogical legacy lives on in the hundreds of his students; among them are Konstantin Simeonov, Odyssey Dimitriadi, Arnold Katz, Vladislav Chernushenko, Yuri Temirkanov, Vassily Sinaisky, Victor Fedotov, Leonid Shulman, Andrey Tchistyakov, Valery Gergiev, Sian Edwards, Semyon Bychkov, Kim Ji Hoon, Tugan Sokhiev, Vasily Petrenko, and many others.