I suppose this might be a musical miracle – pretty much every musician who sees it thinks it is.
The Amazing Slowdowner by Roni Music.
From a variety of digital formats (basically everything in my music library, anyway), you can play back any music, speed up or slow down the tempo, and raise or lower the pitch.
You can, for instance, tune Jimi Hendrix’s guitar to an E, and then slow the song down so you can learn it (anybody who’s tried to learn a Hendrix tune knows what I’m talking about!).
Or you can practice an orchestral piece with a recorded symphony, slowing it down, and progressively bringing it up to tempo.