Going really low tech, Lenco, a Swiss outfit, makes and sells a turntable whose speed is continuously variable from about 16 rpm to about 85 rpm, by the use of tapered drive and driven shafts coupled by an idler wheel. These turntables were sold as part of a Bogen record player for schools and teachers of music and dance.

While this will not retain key signature, unless the record speed is exactly halved,she could certainly hear exactly which notes are being struck as the music progresses.

When I was teaching folk dancing, in a room whose floor was too light to prevent the tone arm skipping across the record as we danced, I frequently recorded the dance music at 1/2, 3/4, and full speed, onto a cassette tape which wasn't affected by floor vibration, so the dancers could get the footings down at slow speed, before they tried them at normal tempo. The same technique would certainly work in figuring out exactly what is being played in any music, especially fast guitar and banjo picking.

Dave