Communication in Codea Runtime with Obj-C Classes?

I have my Codea project running successfully within XCode thanks to the excellent tutorial on

My next question is, how can you call Objective-C code from within your LUA classes? (and the other way round)
Where do you put your functions and how do you call them? (for example, It would be nice to call an Obj-C function that plays audio).

Have a look at

This includes an example to hook up simple audio, playing an mp3 using Objective C.