On phone calls, I have audio if I use the Speaker feature, or if I plug in a headset. But I do not have audio if I use the standard earpiece (when I place the phone against my ear),
I had this challenge with my iPhone 6 a few months after I purchased it. I setup an appt with Apple and they actually swapped out my device.. although now they typically just change the front screen / panel on the device. You prob need to have a repair done. You can setup an appt with Apple via the Apple Store App and pay the fee to have it repaired. https://itunes.apple.com/us/app/apple-store/id375380948?mt=8
e.g. out of warranty repair: https://www.apple.com/support/iphone/repair/ - I think it would fall under the $129 for the out of warranty on the screen repair since the earpiece I believe it tied to the front screen / panel
If you won't live by an Apple Store you could always have a cell phone repair shop fix it.