Stu Harrison from Merriam Pianos reviews PTQ 6 vs. 7

Great, thanks. Love that reviewer.

as far as i know (and i could be wrong because definitely i'm not privy to the details) the piano model is at bottom an applied math model based on the physics of sound production of a piano.  i mean, the underlying model is that of a large collection of vibrating strings coupled to the vibrations of the soundboard and lots of other stuff (like acoustics of the piano cavity).  there are various papers online from people who have developed similar such models.  so any instrument that produces sound by a similar process might be a candidate for a variation on the model (like various tuned percussion, such as bells, xylophone, steel drums, and things like harps and guitars).  but there are plenty of other ways to make sound, and many of them are pretty distant from this kind of model. so i would say that if one wants to go as far as "any recorded sound" then there would have to be some new techniques involved. 

that said, there has been work in training neural nets to generate sounds based on various reference sets.  this is a totally different kind of model, though.  i assume Modartt uses some techniques from machine learning in developing their own models, simply because such things are quite the rage in applied math these days, but i don't know if they do or not.