1. An audience state estimation system comprising: sound-obtaining device for obtaining sound from an audience and generating an audio signal according to the sound thus obtained; volume detection device for detecting a volume of the sound from said audience based on said audio signal, wherein the volume detection device calculates variance of volume of said audio signal for each frame, and averages the calculated variance of the volume in time direction to calculate a time macro volume, the number of frames averaged in calculating the time macro volume being set according to an audience state to be estimated; and estimation device for estimating an audience state based on said volume.