Why is Casey using a #define for DIRECT_SOUND_CREATE? I can see that with the X_INPUT macros he is saving writing the full definitions twice. But DIRECT_SOUND_CREATE is only used once. Is the #define really necessary?

Because he is used to declare function pointers with this kind of macro. It just a habit thing.

But in this case it is not necessary right?

It is not necessary in any case. You can always type full function signature in all the places where its needed. And it will work fine.