Well, DOS in a mainframe was not like MS-DOS!
DOS just stands for Disk Operating System, of which there were many.

My MIDI offerings include Find MIDI Chords, which used to be a commercial program. HARMONY imports abc to a score and exports as MIDI.

My program MIDIWIZ allows
1. Tuning / pitch controlling
2. Transposing MIDI files and slowing them down (Or speeding up!)
3 Control the stereo placement of instruments using a semicircular chart display - drag the instruments to a new location

I have also written a small program for my wife to use on gigs, which lets her set up MIDI playlists, display the words, and also change the key or tempo. It plays the MIDI file through the PC soundcard or through an external sound module (e.g. Roland)

I don't promote that one, but would make it available if anyone wants it.