I installed Cyanogenmod 12 on my D855 but noticed that the microphone would not work with VOIP applications. I looked into this and found a fix which is pretty simple as it only involves changing the build.prop.
1. Using a File Explorer that will allow you to access root files, find the build.prop file in the "system" folder
2. Backup your build.prop by adding .bak onto the end of the filename (just to be sure )
3. Using a text editor, change the line use.dedicated.device.for.voip=true to use.dedicated.device.for.voip=false
4. [I]After you have edited the build.prop as above, restart the device. Changes should take effect.[I]
5. [I]The microphone should have access to VOIP apps now.[I]
I was struggling to find this for a few hours. If anyone has any suggestions or comments, please write them down below. If it worked, say thanks.