We may be talking about two different things. I was thinking of a "cue" whereby the police play the scream, and instruct George to "scream like that." Make an effort to recreate the sound, like one does when practicing a musical instrument of voice passage. Play the original for Zimmerman, record an attempt to recreate the original, play the recreation for Zimmerman so he can compare his attempted recreation to the original, repeat as necessary. In other words, try to make the recreation sound the same as the original, using a feedback loop as a tool.
The only evidence I have that this wasn't done is the disparate duration of the original and recreation screams. I think most people (even children in the 4-5 year old range), given the feedback loop, would get closer to matching the duration than Zimmerman did.
I've always suspected he was instructed for some reason to do it the way he did it.
That maybe they thought he needed to be enunciating distinct little "help me" phrases so they could get a clear sample of the various forms of those two words to compare to the 911 call, when comparing patterns in whatever analysis they're subjecting the sound files to (and I don't really know how this works, other than that sounds can be translated into visual patterns for comparison).
I've got to be honest: if you told me "Five bucks if you scream," I wouldn't be able to do a real scream, just say "eeee" somewhat louder than my normal volume. For some reason, I just can't intentionally do one on request. But if you whacked me in the shin with a baseball bat, I'd definitely be able to scream.
So I can sympathize with someone not being able to scream convincingly when asked to, even if they've got everything riding on it.
However, I've also always thought it unlikely that George actually thought that chain of little staccato "help-me-help-me-help-me"s sounded anything like what he was doing that night. You'd think he'd at least attempt to go "helllllpppp," regardless of how convincingly distraught it sounded.
That's why I think he was instructed to repeat short bursts of "help me".