Why doesn't speakerphone work well when my iphone7 is in the smart battery case?

If I use speakerphone with my iphone7 in the smart battery case, my voice can barely be heard by the other caller without screaming obnoxiously loud.

This is immediately resolved by removing the phone from case and continuing to use the speakerphone, so the problem is related to the case.

I've had this phone & case for about 8 months. I don't know if this is a progressive issue, which may suggest some of the case's speaker ports just need to be cleaned. Or perhaps this is a consistent design flaw with the case. I don't use speakerphone enough to remember a trend.

Apple iPhone 7 Smart Battery Case

Apple iPhone 7 Smart Battery Case

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