The MN31N uses the IGP & MCP-T chipset, the MCP-T contains the Soundstorm audio using the Realtek ALC650 as the interface. The reason that the MN31N is not "Certified" is that it does not have separate jacks for the 6 channel sound and SPDIF In\Out, these are provided for with extra headers on the MoBo but as they don't have the extra audio jacks like the NF7-S its not considered "Certified". The audio itself will sound the same, the Soundstorm software and controls are the same and because its not "Certified" they don't have to pay the extra licensing fees to nVidia which would inflate the price of the MoBo. I've noticed a difference in the sound in games between my boards, my 2 x MN31Ns play all games with great sound and no issues and my AN35N Ultra needed my SBL to play BF1942\D.C. properly. If you were a real audiophile with your MoBo plugged into a digital reciever with a high quality surround sound speaker system I think you'd easily hear the difference between the ALC650 of the AN35N & the Soundstorm of the MN31N. Also remember the ALC650 by itself has up to 16% CPU utilization while running EAX codecs in games but when used with the MCP-T (Soundstorm) it has less than 5%.
Regards, Balrog