If I understand correctly this was a substitution cipher where the alphabet was mapped to notes A-G, 5 sharps and 5 flats (15 characters total)? A-Z0-9 is 36 characters. I guess you could use chords (or different note values or rests) to reach the entire alphabet. Good thing Soviet officials did not insist she (or someone else) play an encoded composition which would probably not sound good.