You can only audition performances from a category search - you'll notice that there is an Audition button on the screen that is not there when you are adding to an existing performance (Performance Merge)

That's because the audition is a predetermined demonstration of a preset performance to showcase the characteristics. Once you inherently change that performance characteristics then the audition function can't reflect your changes so it becomes inactive.

But when I choose one part to add to the other I've not jet modified it, so this justify have no sens to me. The result is that I can't use the audition button in a fondamental moment of choosing.

It's not possible for it to play the audition because a new part has been added which changes the performance as a whole. To choose that additional part you have to be inside a current performance. So if you touch the "+" then you are changing the current performance and audition won't play if I understand correctly.

David I talking about audition button in a category search screen.
I'm modifying a performance and I touch a + to add a new additional part. In this time I see category search page and looking for a new part to add witch is not modified because I take it from its original status, so way audition button don't work in this category search page but only playing keyboard it is possible to listen the new part before insert it in the program that I'm modifying?

When you press + to add a part to a performance the screen you are taken to is not Category Search but Performance Merge.