The HAAC microphones used in the Lumia 620 are the same microphones used for Nokia's widely praised Rich Recording in the Nokia 808 PureView and they are also used in the Lumia 820 and 920. The use of this high quality component on what is a very much a mid tier handset is one of the reasons we rate the Lumia 620 as one of the best overall value-for-money Windows Phone devices currently available.
The key benefit of the HAAC microphones is that they are capable of capturing audio over a broader range of frequencies even at high volume levels (140 dB, compared to 120 dB for most smartphones), resulting in more natural sounding, and distortion free, recordings. The benefits of this will be apparent in all recordings, but are most noticeable in loud environments, such as at a music concert.
Do note that, despite the dual microphones, video capture on the Lumia devices currently only records a mono audio track, albeit one of high quality. This could theoretically be changed in a software update.