Hi,
The "missing mThd chunk" is a problem with the .mid file, that it is not structured correctly to be read and transcribed.
Some media players that don't rely on that structure may be able to open and play the file, so you could try using such an application to re-save the file, though I'm not sure if that will help.
It is in our "to do" list to try and overcome the problem, at least for Notation Composer users, by opening such files in Composer and using the ReBar feature. However, that work is on indefinite hold due to the
unexpected death of Mark Walsen, the president and chief developer of Notation Software, Inc.
If you'd like to send me one of the files that gives you the error, I can take a look at it and see what I can do.
ttfn,
Sherry