My phone mic intermittently works

Hello, I want to know if there's any explanations as to why my phone mic intermittently works. For a while, it sounded like someone turned the volume way down on my mic. That and sounded like I was muffled. So I took my phone in to the closest apple certified technician which was best buy 4 or 5 months ago. They told me they'd just have to replace the phone. But now, like at this moment, my phone mic is picking up everything fine. But an hour ago, it couldn't pick up anything. It makes no sense. It does this all the time. I hold my phone in a way that doesn't block the mic either. (As I understand that would be the first premonition to make) What do I do? Why is it like this?

It's an iPhone 8 plus and besides this mic issue...the phone is the best.