After 2 years of not being able to send voice messages it was time for a change...
I simply took a look on ebay and quickly found a fitting one for about 2 €. It was not exactly the right one with the ideal footprint but at that time I didn`t know better and so i bought it.
The ideal spare part for the lower microphone (LG G3 D855) looks like this:
But you know how things are if there is nothing like a instructable or something else on the internet, you simply have to try out and so did I and bought the wrong one. But hey IT WORKS !!! :D
The LG G3 itself is quite "maker friendly". You can open it very easy. As allways keep in mind, that working on a pcb with microcontrollers without ESD-protection is very risky and can destroy your device, so be careful. ( I have actually no particular esd mat or bracelet but I'm trying to ground myself before starting and between the processing. BUT for further works I'll definitly buy one, I mean they cost about 25€ and can protect you against damaging your expensive devices.)
How to open it is well described in this video: