I have one of those Mathieu metal ocarinas (and another one on its way from an eBay auction). There is a problem with it, which I suspect may be systematic: the topmost notes simply aren't there. You can do quite a lot with just nine or ten, though. (With a spare to play with, I may try a bit of revoicing with a file).

My ocarina page:

http://www.purr.demon.co.uk/jack/Music/Ocarina/ocarina.html