Is the score published under CC licenses? I don't see any indication of such in the score.
Who's Marcus Mota? I suppose he's still alive?
Do you have his permission to publish his scores? If it's not under CC license at first place, and you have permission, you would need to send an email to dbmiller@imslp.org confirming that Marcus Mota allows to distribute the work under your selected CC BY-SA license.
If the files are originally uploaded under a CC BY SA license, you do not need to seek out permission if they were uploaded on another site under the same license. As long as they are uploaded here under the same terms, then its fine.
Keep in mind if a user did not provide an external link to the file of where he or she found it, then its going to be on the burden of the user to ask if he or she got permission if nothing in the file explains where it was originally uploaded.