Holy Moley, you are not the only one!I have found no perfect way to address this; however, I have-sometimes- been able to redirect the energy. Here are some ideas:
1) Give a feedback form to students to fill out during classroom performances. For very young kids, a checklist with pictures will do (ears for projection, a picture frame for stage pictures, etc.) By upper elementary, omit pictures and use short phrases. Middle school kids can handle a more open-ended response form, provided they are given topics to address. The author's name must be on the feedback form, and 'phrasing the comment they way you would want to hear it if you were the actor' always stands as a rule of thumb.
2) To focus the audience- middle school and up, have them write down the teacher side-coach comments. At the end of a class or the end of the project (after all scenes have been performed), code the comments under classification umbrellas (voice, movement, etc.) Have the class determine their ensemble strengths and challenges from the totality of your side-coaching comments.3) Teach the kids to take on the job of side-coaching while you write comments. (This one takes a lot of trust in your actors.) After modeling how to side-coach, scaffold the skill by picking a student to side-coach, and give him or her one specific area on which to focus comments (it will take awhile to develop language and deliver comments when you are not also giving the 'real' comments.) The audience will be interested in what the novice side coach is saying, lessening their aloud contributions. They will also be thinking of how they can phrase things if it were their job.... as well as watching their peers.
Those are a few that I have had some success with. Good luck. It is tough to break the kids out of engaging in the practice you are modeling regularly. (If they are doing it well and with good intent, it also says a lot of good things about the relationship you have with your kids.)