There is a great video on You Tube of an actual nuns' choir doing the same piece as in the Sister Act film. It has great shots both of crowd reactions and the musicians as well as the singers, so you really get a sense of how the music is affecting everyone (and they're not actors!). You can find it at: link removed no longer available due to copyright violation
Moderator update: found this video on YouTube, it's not nuns singing, instead it's André Rieu & His Johann Strauss Orchestra performing "I Will Follow Him" live with the Maastricht choir (I believe). But it does definitely show the audience's reactions.