I changed the Lightning Connector Assembly and microphone is working fine now and no more crashes. Bought it on eBay, new and original from Apple for 12€. Here ist the guide from iFixit: iPhone 7 Lightningコネクタアセンブリの交換 But i used this video: https://www.youtube.com/watch?v=KMh_lOZT... It took some time because nearly everything, except the battery, needs to go out first but there were no problems doing so. The only important thing is to store each screw and part in order so you can reassembly it backwards.
So i called the Apple Service. First off the pixel glitch is reported by many many users in the internet and seems to be a problem from Sierra. Nothing crucial at least. With the Apple Service we deleted and recreated the Fusion Drive. The iMac seems to run very fast now: First we startet Internet Recovery (If i remember right press CMD+R after starting the machine). Then we opened Terminal and deleted the whole Drive including recovery stuff. (1) diskutil cs list -> Copy Logical Volume Group UUID (2) diskutil cs delete <Logical Volume Group UUID> (3) diskutil list -> Copy Logical Volume Group UUID again and find SSD and HDD id's (4) diskutil cs create Macintosh\ hd disk0 disk1 (in my case disk0 = SSD, disk1 = HDD, found in previous step) * diskutil cs createVolume <Logical Volume Group UUID> jhfs+ Macintosh\ hd 100% Then close Terminal and start installing macOS. (In my case only El Capitan was available and i upgraded to Sierra later)
Yes: I sold it on eBay and bought we new one... :/ Update (21.03.2019): Exactly the same happened to my Macbook Pro, 15”, 2015. The Apple service provider changed the logic board 3 times. The third time i didnt took the Macbook home, we could reproduce the issue immediatly. In the end Apple gave me a brand new 2015 Macbook Pro.