Why have I lost stereo sound on my iphone 5 after installing ios7

That's because one of the "speaker grills" at the bottom of the phone is NOT a speaker. It's a microphone. It's always been a microphone.

Since updating to ios7.04 I only get headphone sound on one channel. Multiple headsets so can't be the jack...

Settings/General/Accessibility. Scroll down to the balance slider and move it to the center.

It IS centered. Im saying its not working when it prev did and I havent changed any settings...