Let’s say the auditorium fits 500 people, that each person takes one poo per day and one poo is on average 300 g, and that the rate of digestion is even throughout the day. If there is a uniform distribution of people based on stage of digestion, the average person will have 150 g of poo in their large intestine. That makes 75 kg of peo in the poople in the odditorium.

Further details, like what time of day it is and how many people are morning poopers vs. evening poopers and how fast their metabolisms are, might offset this by a small factor, but it’s probably not going to be more than 125 kg or less than 40 kg.