This sounds most likely like a bottom microphone problem. As was already noted, I would try and make certain there’s no debris stuck in the bottom grills. Although it was mentioned to use compressed air, I would recommend using something like Blu tack, or even tape to pull stuff out since that’s generally more effective than trying to blow it inward.
If it looks clear, it may be the microphone itself is not working properly in which case you’d be looking at replacing the lightning dock assembly.
[product|IF371-005-4]