Formal:
* Discography: This is a common term for a collection of recorded music, especially by a single artist or band.
* Catalog: This emphasizes the organization and listing of the music within the library.
* Archive: Implies a collection of significant or historical music.
Informal:
* Collection: This is a broad term for any group of things, including music.
* Repertoire: This term is often used for a collection of music that a musician or group performs.
* Sound library: This emphasizes the audio nature of the collection.
* Playlist: This refers to a curated selection of music for listening.
You could also get creative and use a metaphor:
* Symphony: This evokes the harmony and richness of a diverse music library.
* Chorus: This suggests a unified and harmonious collection of music.
* Constellation: This implies a collection of diverse and interconnected musical pieces.
Ultimately, the best collective noun will depend on the specific context and the type of music contained in the library.