I'm experiencing this too. No solution. Brand new iPhone less than two weeks old. Overnight Siri stops working. Called AppleCare who tried to convince me to pay £30 UPS and stump up the cost of full iPhone incase I didn't send it back. Told them this was unacceptable and wound up talking to a senior support technician. I've tried everything they suggested + everything on here minus taking the front off and cleaning the mic.
I've logged feedback and I'll be calling them back to get a replacement. I'm still in warranty, so they can insist on policy all they like, they're sending me a new one.
I've had replacement Apple Products ship out for free several times before.
My mate had two faulty iphone 6s, one had terrible back-lighting issues and another had a faulty camera.
Might seem like I'm angry here. I'm really not, just disappointed. My last phone was the 4 with that dreadful loss of signal issue and really I haven't had much success with Apple ever since I started using them about ten years ago. Actually my original ipod was great. Foxconn are really bad. One last go and maybe it's time to jump ship and buy myself a Korean manufactured phone for half the price that does everything that Apple are getting around to doing in their next OS.