The cassette tape Kathy found in Norfolk was the exact same copy she lost.
Ruth wanted a way to repay Kathy's previous kindness with the pencilcase incident and decided that replacing the tape if she 'lost' it would be a good idea. She took the tape herself and hid it with her own collection.
When she gave her collection to Keffers at the cottages, he actually did bring it to a charity shop...in Norfolk, where Kathy and Tommy found it years later.
The Madame stole Kathy's tape and kept it in the gallery.
It dissapeared right after the Madame saw her dancing like that, and it summed up so perfectly
exactly what they were trying to prove about the students having souls that she knew it had to be in her gallery.