Once the GDR2/Amber update is installed on your Nokia Lumia 920, you will need to install the Nokia Pro Cam app. The phone sports three high amplitude audio capture microphones and only one of them has been active for recording until now. One has been used for noise cancelling.
To compare the mono and stereo record abilities on the Nokia Lumia 920, you will need to listen to both YouTube videos below. The first one was recorded in mono with stereo used on the second clip. The stereo recording feature could end up activated on all Nokia Lumia 92x models.
source: YouTube (mono), (stereo), MyNokiaBlog via WMPoweruser