Chosen Solution
As stated in the title the bottom mic is not working after I replaced the whole bottom connector unit with a 3rd party one. It appears to me as if the mic is not in line with the whole in the case. I cannot aline it properly as its position is pretty much fixed by the cable it is attached to. And idea what I could do? Are there faulty connector units (construction-wise) on the market? Also a little rubber seal (I guess) got lost at the replacement. Not sure if that was part of the mic or earphone jack. Update (10/13/2017) Thanks for the answers. I fixed the problem now: I bought a used back-case on ebay an took it apart. As suspected, between the mic and the hole in the case there is a small plastic piece which guides the sound from the hole in case to the mic which is attached to the dock connector unit. I do not mean the mesh part. When I removed the old unit this plastic piece stuck to the unit and I accidently threw it away. In the video you posted the plastic thing seems to remain in the case. So when anybody has a similar problem. You might wanna check the old unit if there is still the plastic piece stuck to the mic part.
Small parts, such as flexes, cameras etc. have a reputation for low quality. It is entirely possible that you have a defective or low-quality replacement flex. Ideally, you need either a known-good component or have access to another working device to help isolate the problem. I wold also go through this repair guide, if you haven’t already done so, to insure everything looks good.
