Voluntocracy Music License and claim that MIDI files are recordings, and as such have copyright status independent of the transcription.

The abc2midi program renders a MIDI file using only the ABC file as input. Thus I assume the resulting MIDI copyright would default to either the abc2midi author or to me. The abc2midi program is GPL2ed and lays no claim to the output:

The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program). Whether that is true depends on what the Program does.
Richard Stallman suggested license wording which I have applied to all these tunes. The license appears at the bottom of the sheet music; and abc2midi puts a copy of the license on every track.
Copyright (C) 20xx Voluntocracy.
Permission is granted to copy and distribute modified or unmodified versions of this music provided the copyright notice and this permission notice are preserved on all copies and the entire such work is distributed under the terms of a permission notice identical to this one.

Folk Music Voluntocracy
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Go Figure!