iphone 5 no audio from speakers

My iPhone 5 suddenly lost its audio function. There is no sound from the speakers: no music, podcasts, ring, ringtones, game sounds, etc. Rings from incoming calls do not sound. Voice mail cannot be heard. When I speak to someone via the phone they hear me, but I cannot hear them while using the phone's speaker.

When using the audio-out jack I DO hear sound. Also, the phone's vibrate feature works.

This loss of audio occurred suddenly while I was speaking at a desk, not moving. I then checked the Apps, and I found I could not hear their audio output from the phone's speaker. I could only hear their output from the audio-out jack.

I have restarted the iPhone 5 several times.

I restored the software on the iPhone 5.

I am using IOS version 6.1.2 (10B146).

Has anyone had a similar phenomenon?

Any suggestions to resolve this problem?