The problem is easy to see if you look at the abc for the file. They've used "A", "B", "C", "D" to identify the parts which is not the correct thing to do in abc. This usage makes them chords: A, B, C, D major. They should either have used "^A" etc to put the letter above the staff of used the P: keyword to identify parts properly.

When the midi is generated from the abc it converts those "A", "B", "C", "D" to the accompanying chord which you hear - not particularly sympathetic in the key of G

Mick