Looking through the GPL FAQ, there is the question of if you can use the GPL license for non-software products, as long as it is clear what constitutes the source code. Of course, that does not mean it is advisable to do so. Asking mostly out of curiosity:
I thought about licensing original sheet music under the GPL license, and providing the source code along with it. If I were to do this, I would use Lilypond to generate the sheet music and since Lilypond is open-source and free there is no problem with this - anyone can access and modify the Lilypond code. And I like it that way. But what if someone took my music, but for whatever reasons did not want to use Lilypond to modify it so they translated it into a more common but non-free software such as Finale. As such, their "source code" would be the finale project file, which as far as I know can only be opened with Finale which requires a fee for use (this may not be true, but for the premises of the question we will assume it is, since this is more of a conceptual question than a question about these specific details). It seems that providing sourcecode that can only be accessed or compiled with nonfree software violates the sprit of the GPL, but what about the letter? Are they required to translate their code back to Lilypond for distribution as required under the GPL, or does the Finale file still count even though a fee is required to even open it? As an in-between, what if they translated it into a program like Musecore that is free and open source but requires entirely different procedures to modify (Musescore is graphical editing, Lilypond is actual code. Sort of like LibreOffice and Latex).
Edit to address some questions in the comments: To my knowledge, Finale project files cannot be opened without paying to use the Finale program. Please assume this is true for the purposes of this question, since this is more of a theoretical question with a specific example to explain the theory, than a question about the specific example.
Note that this restriction would be beyond what is required by a Creative Commons Share Alike license - that requires that the new thing made based on my thing be shared under CC-SA, but not that the files used to create it be shared, let alone be in a modifiable format.
If GPL does not require that any derivative work of my music is released in Lilypond code or at least something free to modify, is it possible to modify it to make that be required?
Related, but not duplicate, since in this case the provided source can be compiled, just not for free: Do you violate the GPL if you provide source code that cannot be compiled?
Vaguely related, though in this case I think what the source code is is clearly established: With GPL, what does "source code" mean for non-software?
Near duplicate, but not quite, since it is not about the format of the source files, only their existence: Is there a share-alike licence for media which specifies that the project files must be published as well as the finished media?
But, as stated, I do not work in the music industry and only write music as a hobby. There are free tools out there, and several of the are quite good. Musescore is the most popular, but it has become increasingly locked down - the software itself is free but access to other's music published on the website is not, there is some sort of pro subscription that gives you something or other, etc. It also is a graphical WYSIWYG type program, which I find annoying to deal with because it is hard to copy and paste and is generally a destructive style workflow. So I use Lilypond which is a plaintext based program that, for lack of a better comparison, is basically LATEX for music (I think it even uses a modified TEX engine on the back-end to make the PDFs).
But that is not the point. While I do not care about performance or recording rights (or at the very least, they are outside the scope of the question), if I ever decide to publish something I have written, I would like to require that any published derivative work have the music and source files shared in an accessible format. This is to avoid anyone wishing to make a derivative work of the derivative work being forced to transcribe from the PDF sheet music into Lilypond or another program, which is tedious, time consuming, and error prone.It is much easier to give them the file used to generate the sheet music so that they may modify it directly. Apparently this is an uncommon thing to do, so it is rather hard to research the concept, let alone determine what if any standard exists for this.