Q: How do I adjust volume on iPhone 5 ?

The volume was OK on a call.  The next call I made, volume fell off to nothing, and sounded very tinny.   The only way I could hear the other party was to switch to speaker phone which was OK.  I had the volume turned up all the way.

I've tried re-setting phone with no luck.  I have some plug in earpieces, and they're OK.  I also have a Bluetooth Planotronics earpiece that is OK as well.

Where do I go from here?

Thank You...