Voice is really muffled in calls. Get's fixed when using speaker.

My mom had this problem with here phone for quite some time now that here voice is really muffle when making a call. However when switching to speaker the voice gets much clearer.

So I did a bit of googling and it seems like the noise cancelling mic on the top is the problem. Though I haven’t found a solution for it yet.

It has no warranty so that’s not an option. As far as I can see the noise cancelling mic is part of the motherboard, so in case of a hardware defect the whole thing would have to get changed, right? In case it’s a software problem, how would one go about fixing it?

Thanks in advance!