Right, Ed.... mp3 / audio to MIDI is waste of time...
The process is similar to converting a bread back to the ingredients... flour, salt, etc.
With Yamaha styles the analogy isn't really appropriate.
PSR-500 is different that the later models, but conceptually this may be possible, depending on finding an alternative way of selectively muting the tracks.
The Yamaha accompaniments are 8 separate MIDI tracks, each consisting of a rather well-defined instrument. Additionally, the CHORD, BASS tracks will play only subsets or transpositions of Cmaj7 chords. So the audio produced by the PSR may be actually parsable by some computer program.
I have a PSR-E363 and as someone restarting my music education after a long break, I find most Yamaha accompaniments too complex sonically to properly understand the harmonies applied. Unlike higher models, like the PSR-E463, my box doesn't officially support muting or mixing tracks. But I've found that the tone generator actually does respond to the MIDI control messages allowing me to selectively quieten the various parts of the octet that plays the accompaniment.
Somewhat similar approach may work with the PSR-500 and the computer sound analysis.
Edit: spelling and grammar fixes.