OK, something fundamentally changed in the last weeks regarding the access to songs in the iTunes library. I do get support requests every day and we also have multiple threads in the forum about this. I will write a big FAQ post why this is happening, what to do when it happens and how to prevent it in the future.
Just to be clear, this only affects version 2 of Stage Traxx. Because of these problems, version 3 copies songs from the iTunes library into the app. And the only way to see missing songs in version 3 is to delete the audio file from the Stage Traxx 3 folder yourself.