For the record of this thread, I want to restate what I previously posted concerning turning Hearing Aid Mode on in Accessibility to increase the phone's volume. This actually does work. It did in mine and my Wife's phone. She has excellent hearing, and when she got her new 5c last year, she couldn't hear it. The volume was about half my iP 5. I did a search and found where it said Hearing Aid Mode turned on could increase volume. So I tried it on her phone. It worked. She was able to hear loud and clear. I then turned it on for my phone. It did increase my volume some. But not as much as it did on her 5c. Your mileage may vary, but if your iPhone doesn't seem to have the volume it should, it won't hurt to try this. It worked for us!
Sent from my ancient but trustworthy iPhone 5. ☮