I'm experiencing the same problem. Tried everything, microphone does not work with any guest OS on any version, on either Intel or AC97, on any paravirtualization interface. I tried it all. Also tried ALSA, Pulse and OSS.
Thought it could be a problem with pulseaudio, so I tried direct pulse and jack with pulse, no change. Tried Ubuntu Mate and Manjaro, no change.
I checked and there's a lot of changes in the changelogs for recent Virtualbox in the audio department. Apparently they broke audio input and probably have not noticed or been able to fix it.
I just tried Virtualbox 5.1.6. Mic is working again. Yay! I have a Windows 10 guest. At first, mic input from host device was still going to CD Audio in the Windows 10 guest VM instead of the mic. I had to uninstall the HD Audio driver and let Windows 10 detect the audio card again. After reinstalling the driver, the host mic is now properly mapped to the guest mic.