I believe that Gospel Library for Windows follows the paradigm of having sheet music within Gospel Library, whereas Android and iOS have a separate app (LDS Music). There are use cases for both, but this is how I see it:
Gospel Library is focused on gospel reading and gospel study. There are annotation tools, you can highlight, read, and cross-reference between scriptures, talks, and the text of hymns. When it comes to gospel study, there is little need for the sheet music, as it is non-highlighteable and non-cross-referenceable.
LDS Music, on the other hand, is a replacement for a hymnbook, and a music player. Ideally, bringing up a hymn should be as quick as pulling out the hymnbook from the bench in front of you, without navigating through menus and submenus of content.
Music content is different from other content in several ways. For example, playlists, or multiple audio files to choose from for each song (instrumental vs. vocal). I don't think these fit in as well with anything else in Gospel Library, without over-complicating Gospel Library. Having a separate app allows you to fully develop features that have to do with music.
Samuel Bradshaw • If you desire to serve God, you are called to the work.