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.
Here's my logs, just for the record: