Sounds like there might be an issue with one of the microphones. If you go to this part of Apple’s website you can check to see if your device is still within its 1 year warranty or if you have apple care and they should be able to take care of it for free.
https://checkcoverage.apple.com/
If its not under warranty and Apple won’t fix it for a reasonable price, call around your local repair shops and see what they can do.