The NoteWorthy (NWC) file format is good for printing and for audio/visual playback. It is highly recommended that you get comfortable with using this format. Use NoteWorthy Player (free) to view and play these files. Use NoteWorthy Composer (evaluation version) to view, play, print, and edit these files.
The PDF file format is good for printing. Use Acrobat Reader (free) to view and print these files.
The MIDI file format is good for audio playback. Use just about any multimedia player (e.g.Winamp) to play these files.
The MP3 file format is an audio recording, typically of some group performing the song. Any multimedia player will be able to play MP3 files.
We may eventually provide some recordings of the "nugget" group singing their songs, so that "remote" participants in those songs can get an advanced feel for what the tone and style of the performance will be (rather than having the remote participant's first exposure be, say, on the morning of May 15). If we do ever provide such recordings, they will probably be in MP3 format, which any multimedia player will be able to play.