There is a good reason behind sound proofing the entire pit. They want the sound to come out of one central location ( the speaker system.) But, if your sitting close enough to the pit you would end up hearing it from two places the pit itself and the speakers.
It is like if your in the front row of a show you end up hearing what is onstage from two locations. The actors up onstage and the speakers as well.
And as for this going on at The Little Mermaid. I am thinking it might just be for the saftey of the kids that could be in the front row. Like they could accidentally look down into the pit and fall over the bar that separates the front row from the pit. When Beauty played the Lunt I remember there being a net there instead of the plexi glass box.
But, I have heard of the use of the pelxi glass to help block sound and have it go to one central location.
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Tom Stoppard's Rock N Roll