The phrase "full auditorium" means that all the seats in the large room where people gather to watch a performance, lecture, or event are occupied by an audience. Full definition
«They're having massive protests over there,» Trump told a mostly full auditorium in Winston - Salem, N.C. «We had a great convention. (scientificamerican.com)
He slipped in unannounced and it took a while for the almost full auditorium to notice him. (theconversation.com)
Speaking to a nearly full auditorium about the college's financial crisis, board chair Mark Epstein opened by saying there was no culprit, that «the problem is a systemic deficit.» (nplusonemag.com)