IMO it means the front mic part of the front camera flex cable is not *recording* sounds like it should. It could be that A/ the mesh covering the ear speaker and front mic is clogged, or B/ the cable is improperly seated, or C/ the mic or cable carrying it is damaged.
I'd start with A/, pierce the mesh of the ear speaker with a fine needles ~3 times in the middle of the mesh. Don't let it in more than ~1mm to avoid disturbing anything in there. Then test the mic.
B/ If no improvement, I'd use this guide until step 11 to disconnect then reseat the front camera cable. This may help, although not very likely.
C/ Change the front camera flex cable following the guide here above all the way to the end.
Note: if issue is a clogged mesh, it can be changed using that same guide here above to remove the front cam flat cable (like in option C), after which push the mesh in, from the outside towards the inside of the screen using a flat plastic tool. Get a replacement mesh from eBay or just peel off the top layer and put the rest back in. It's a bit of a pain to replace that mesh and there is always that inherent risk to every repair to damage the phone or screen.