I also am involved with technical theatre. You're question I think is asking if you could hear them singing. As tiawah466 said, you proabaly would have to be so close, that you would be able to hear it anyways. Also, if they are using backstage communications, (unless its actually radios with a good range on them, which is unlikely), you probably wont be able to pick that up anyways. Only the higher priced scanners can pick them up as I've seen. At the place where I usually work, we use these headsets from Telex.
http://www.telex.com/Intercoms/products.nsf/pages/btr300!OpenDocument&c=files
They have very poor range. So poor, we can't even use them in our auditorium, only in our small theatre. They operate in the 150-216 mHz range. The reason for using them instead of a traditonal radio is to allow mutilple people to talk at one time. For example, If there was a problem being discussed, and the stage manager needed to call a cue, he could talk over them (hopefully they would then stop talking...), and be like "Hey!, Standby!!". As you can see on the website, you can get pretty advanced with stage communications, including encrypted and higher frequency ranges (518-608, 614-740, 796-868). Hope this helps...haha..probably too much information...