It's not necessarily an easy question to answer, at least for the general case. And bear in mind that I'm not a copyright lawyer, I'm just interested in the area.vlint wrote:If the song itself is old enough to be public domain (and the midi is not an arrangement), is the midi also public domain?
Essentially, every time someone makes a 'creative work', they have copyright over it, and you need to wait for that copyright to expire before that thing is public domain, regardless of what sources the creator drew from. So if, say, T.H. White writes a novel based on old writings about King Arthur, you can't say that just because those old original works are in public domain the new novel is as well, because White expended considerable creative effort to write his book. However, if you photocopy an ancient copy of L'mort d'Arthur, then you don't gain copyright over the photocopy because you didn't do anything creative, it was only a mechanical process.
So technically, I believe the answer is that if they perform a straight transcription from a public domain source into MIDI, from a prescribed tempo on a prescribed instrument, making no choices themselves as to the arrangement, then yes - the MIDI file is in the public domain also. However, if they play the music by hand on a MIDI keyboard into a sequencer, then they've definitely got a performance copyright; if they select a tempo or an instrument or fiddle with the arrangement of the notes at all or make any other creative decisions between original music and MIDI file, then they own a copyright over their interpretation of the piece.
So yeah. The safe option is to transcribe/play it yourself.
(On a totally unrelated note, any recommendations for sites for celtic folk music? I'm interested myself, I've been working my way through the sheet music presented here: http://www.cpmusic.com/tradmus.html , but I'm certainly interested in any other sites with collections of similar styles...)