Just dropping in my 2 cents (I posted this on the Australian Whirlpool forums like some others here). I originally had a launch day 7+. It had the static/hiss which was being picked up in normal video, and was clearly audible and got louder during any sort of load on the CPU. I contacted Applecare, who went through the process of requesting two videos showing the audio problem, escalated it to a 'senior engineer', who then advised to replace the phone. I returned for a refund instead.
Last week I bought a new 7+ on a whim and got a Week 40. It definitely has no hiss. I've recorded video, run Octane and 3D Mark Sling Shot extreme (twice), downloaded all my apps, etc, and I can't hear any sounds out of the back of the phone, even in a silent room.
It sucks that there is such a broad mix of iPhones out there with varying degrees of quality, and this whole process is a lottery, but I wanted to assure people that non-hissing iPhones 7+'s do exist.
Good luck with your phones. Juts remember, your paying top dollar for these iPhones. If you don't like it, return it and try again.