Several audiences left before the film finished.
Audience is indeed countable but it is a ‘collective’ noun, and so an (=1) audience can comprise many people. If you want to focus on a subgroup of an audience then it is common to refer to these people as ‘members of an audience’:
An example of audiences (plural) might be:
In this case the same film was watched simultaneously by many different groups of people (audiences) in many different locations.
I’ll end this post with two illustrations. The first shows audience, the second audiences.