In total, more than 50% of the audience was unimpressed by the play.
Because "50%" clearly refers to a single number, was (the singular form of "to be") is technically the correct verb. But for any audience larger than two people, the intended meaning is clearly plural, and thus, the verb should be were.
The latter solution is generally considered correct (the jargon for this is notional accord), but in practice, neither solution is ideal: no matter which one you choose, some proportion of your audience will consider the verb accord to be wrong. This suggests that a better solution would be to avoid the problem: "more than half of the audience members were unimpressed". The use of more than half combined with audience members in this minimal edit makes the meaning more clearly plural.
This solution indicates yet another service editors can offer: the ability to pick a solution that is technically correct and satisfying to all your readers.