Most of the tunes in DT are in SONGWRIGHT format - there are a few in MIDI format, but I don't think these actually are used from the new version of DT. According to another thread the new DT plays tunes by passing MIDI events (ie individual notes) to QuickTime. This is presumably so that it can synchronise the note with the moving dot.

If you were to try and play one of the SONGWRIGHT tune files directly, Quicktime would probably not recognise the file and try to find a "Codec" for it.

If you do want to play a tune file directly, you can do it by using the SONGWRITE "SING" software that came with the DOS version of DT, or the "MUSICEASE" software available. Otherwise you can save the tune in ABC format from the DT screen and convert that to MIDI. Obviously if you have a full version of SONGWRIGHT, you can use that to produce a MIDI file.