Q: Cannot hear clearly through receiver of iphone 5c with volume turned up

See this support article: If you can't hear a person on a call or voicemail or if the sound isn't clear on your iPhone - Apple Support

It likes like you have done all the recommended the steps. The last suggestion is to contact Apple Support. https://www.apple.com/support/contact/

-AJ

You are welcome. The factory restore is worth a short, and you have nothing to loose.

Will be interested to hear if you tried factory settings and if it worked.  Since day 1, I have had a lot of trouble hearing people on my 6Plus and had NO trouble hearing on my old 5.  Ridiculous.  Have also had people complain about being able to hear me.  I'm on my fourth 6Plus so what does that tell you???

If you rule out a software issue with the restore, then it is likely one of a couple of things. Either your receiver needs to be replaced (liley if you happen to work around metals). Or your earpiece grill needs to be replaced. The earpiece grill consists of a few layers, and even if it appears to be clean, it can still have stuff stuck between the layers. This causes the sound of the speaker to be either quiet, or distorted, or both.

Apple won't replace just the earpiece or the earpiece grill. They will likely offer you the option of taking your old phone, and selling you a refurbished one in return. Your alternative is to take it to a good independent repairer who can replace either of those components for you. This option does mean forgoing any post warranty support from Apple.

Thanks for the update. It would cost you about $20-$40 at a repair shop. It's a very simple fix.