thats a very ambiguous statement with no true means of giving it proper weight. Actually the reference is to the background noise in a theater. If there is no background noise and if the movie is not playing the sound level would be very low, like in the sound of a fan (at most) level.
Regardless, unless somebody has a dB meter in their pocket it will be extremely difficult to convict on such a law that requires a factual statement. The intensity is highly dependent on the distance from the source as well as consideration given any interference between the source and the reciever. Even the amount and direction of air will alter the intensity of the sound.
I do not see 9-10 people as necessarily indicating any particular level of sound. The fact it is more noticeable when it is the only source of sound does not equate to it exceeding the states limits.

