If you are looking for noise made by people in a concert hall (sighing, page turning, feet shufling, chairs squeaking etc.), then you can get it here: http://dummyworld.net/programs/?category=Music
(look for Kontakt 3 Shhh effects library)
If by "ambient noise" you mean something like what a famous Russian actor recently described in his interview, saying that there are two types of silence in the theater: a silence of an empty hall and a total silence of the hall filled by the audience in the state of awe, their attention drawn to the stage, then I guess it can hardly be percieved or captured by anything other than the minds of the people present at such an event.