Why different kinds of ceramic caps?

I'm aware of the reasons ceramic disc cap should be avoided in the signal path (but may be unavoidable in other locations).

However, in browsing the Digi-Key catalog (an activity which the which spouse thinks is majorly wierd) I notice several variations on cermic caps such as "monolithic", "layered" etc. Should these be catagorized pretty much the same as the more common ceramic disc or do they have properties that are sufficiently different that they are in a separate category? If so are those properties of any interest from an audio point of view.

This is hardly an urgent question, I just want to satisfy my curiosity.
 
I would generally characterize monolithic or multilayer ceramics as better than discs for virtually all applications (excepting perhaps high voltage?), but they probably have limited value in analog audio applications.

There are so many variables with ceramic caps, including gross construction (disc/multilayer/single layer); individual manufacturer's form factor, construction and termination choices; and the many dielectrics available. The characteristics of NPO, X7R, Y5V, Z5U, F, Taiyo Yuden's low-distortion "SD", etc all lead to very different capacitors.
 
This topic is very interesting but from a professional view. I have tried to study the pros and cons of different dielectrics. I use XR7 always as decoupling capacitors (SMD) and I can't see the real difference between the types as long as you are not going to use them in critical places.

If we talk small caps (<1nF) I don't see why I not should use NP0.