I had dropped my phone, and I was in the middle of a phone conversation … I could still talk to the person, and I could hear her … but after our conversation was done, I tried to call my daughter. She could not hear me, and I could not hear her. Texting and everything else worked fine, but even the noise as you type texts into the phone was muffled. I slapped the phone a couple of times on my hand, and the texting noise came back, but still the audio on the phone would not work.
After discovering my problem was also somebody else's problem online, they fixed it this way:
go to Settings>General>Reset>Reset Network Settings.
Apparently this resets back to the original network settings, but nothing changed on my phone -- except I HAD MY AUDIO BACK!!! Didn't have to go to the Apple store, didn't have to break into my phone to attempt to fix it manually, just had to push a few buttons. Phew!!! No need for a new iPhone!