The fictional cosmic music that begins creation in J.R.R. Tolkien’s The Silmarillion is Ainulindalë.
Ainulindalë, whose title means “The Music of the Ainur,” is the opening section of The Silmarillion. It describes how Ilúvatar, the supreme creator, gives themes to the Ainur, spiritual beings who develop them into a vast music. Melkor introduces discord, but Ilúvatar incorporates even that rebellion into a greater design.
After the music, Ilúvatar reveals a vision of the world, and some of the Ainur enter it as the Valar and Maiar. The account establishes the theological and mythic framework for Arda, the world in which the later histories of Elves, Men, and other peoples unfold.
Music of the Ainur is an English translation of the title, not the canonical Quenya title usually requested by the question. Akallabêth is a separate section about Númenor, while Dagor Dagorath refers to a prophesied final battle. Ainulindalë is therefore both the title and the name most specifically associated with creation through music.