ABC is the other major player in the space besides MusicXML and Lilypond.
The Mutopia project [1] is magnificent but largely abandoned in favor of MuseScore [2] which uses MusicXML. The amount of music available there is staggering but locked in to their backend.
Unfortunately automated translation is difficult because Lilypond is very oriented towards specifics on how the notation is rendered.
And, if you are interested in general archives of music, IMSLP [3] (Internet Music Score Library Project) has to be mentioned as well, but mostly contains scans of existing engraved music.
The Mutopia project [1] is magnificent but largely abandoned in favor of MuseScore [2] which uses MusicXML. The amount of music available there is staggering but locked in to their backend.
Unfortunately automated translation is difficult because Lilypond is very oriented towards specifics on how the notation is rendered.
And, if you are interested in general archives of music, IMSLP [3] (Internet Music Score Library Project) has to be mentioned as well, but mostly contains scans of existing engraved music.
[1] https://github.com/MutopiaProject/MutopiaProject
[2] https://musescore.com/sheetmusic
[3] https://imslp.org