We recently started evaluating songs for their quality for use with the StringClub player tools. All songs on StringClub start with a midi, but not all midi files are suited for use with StringClub and some may be decent but not the best choice for someone who hasn't used StringClub before.
As such, we've now started to keep songs that appear to be lower quality out of the main song directory and unavailable to non logged in users. Those songs will still be available to logged in users and visible in the uploader's song upload page. This is part of a larger effort to manage the quality of songs that appear in the main song directories and visibility to new users who have no experience using StringClub.
If you feel a specific song has a lower quality score than it should, please let us know the specifics so we can take a look at that song and see what's going on.