I tried the English UK thing but it didn't work at first.
It only worked after I set it to English UK and restarted my phone and then changed it back to English US and restarted my phone.
After the restart when I changed back to English US my 5c is loud again.
I also took a qtip and cleaned the gunk out of the ear speaker and got even louder sound.
My qtip was black on the end.
I used alcohol on the qtip.
Don't understand the fix but by golly it works.