I'm going to assume that you have done a power cycle on your phone just to rule that out.
It sounds like your receiver (listening speaker) is out. I would try putting it on speaker phone and see if that makes any difference, if not, it sounds like a hardware issue.
However, just to be completely rule out software issues, I would backup your iPhone then wipe it clean and do a fresh install of the iOS. If it works after that, then migrate your information back and see if it continues to work. If not, then you're dealing with a hardware issue and will need to consult an Apple store or an AASP (Apple Authorized Service Provider)
Best of luck!