Q: iPhone Sound sent to imaginary headphones

I used a pair of bluetooth headphones this morning with my iPhone. After unplugging them, no more sound has been coming out of my iPhone. I played with the settings, I rebooted, but no more sound when I call, when I FaceTime, when I play a video on YouTube, no more keyboard sound, no more text alert, nothing. The only sound coming is the ring when someone calls me. That's it.

I realized then that the phone is still sending the sound to "headphones" (for example, checking my airplay while playing YouTube), while my bluetooth if off, and no headphones are plugged to the phone. And there is no option to redirect the sound to "iPhone". It's like the phone forgot its own existence in term of output for sound.

Any idea that could help?

Please note that I upgraded my iOS about 2 days ago to 9.3.4.

Thanks

Laurent