Thanks for your help, but im a bit confused now. Calling in earpiece or loudspeaker mode does use the bottom mic like flopjoke wrote and the back is for noise cancellation only?

Wikipedia says so too: "The N9 has two microphones and a loudspeaker situated at the bottom of the phone. The main microphone enables conversation and recording. The second microphone is located on the back of the device near the flash LEDs and main camera, it is used by MeeGo system for noise cancellation which make phone conversations clearer in noisy environment."

But then again, it cannot be an hardware issue, since it works flawlessly calling in earpiece and stops working completely in loudspeaker mode, am I right?