I recently had this problem and it frustrated me for quite awhile until I finally figured it out yesterday. I have a third party charger that I think caused this problem. It has locks on the left and right that lock the charger in place and I think at some point I've pulled on it without fully depressing the buttons causing me to slightly pull the connector out of place. To fix I took a flat object and lightly tapped back in the white piece that connects to the charger on the phone. I know this sounds ridiculous but as soon as I did that my Airplay options went from "Dock Connector" to iPhone and my sound started working once again. I don't know how long this fix will work, but it has been a day and so far so good.