Hi ravishan12, from time to time windows just fails to install driver software correctly.
(Not uncommon). There's more than one way to get your sound driver software installed.
Make sure the bios is set, AZALIA AUDIO [Auto]. See page 2-22 in your manual.
Though I know you have an Asus CD with the sound driver on it, I don't want you to use it.
Create a folder on your desktop & name it "Sound". Go to Asus & download the sound driver into your "Sound folder".
Click on this Asus M2N-MX link & choose your operating system, than click on "M2N-MX related files for download". Click the plus sign next to Audio & download into the Sound folder.
The file will be in .zip format. Open the Sound folder & uncompress the .zip file - keeping the uncompressed files in the same Sound folder. Once uncompressed there will be two folders. One 32bit & the other 64bit. (This refers to your operating system) Windows 32bit or Win 64bit).
If you do not have the program, "Winzip" to uncompress the .zip file you can download it for free at this next link: http://www.download.com/WinZip/3000-2250_4-10003164.html
Next, how to get windows to install your sound driver software without failing. Open windows device manager. Scroll down through the list of icons. You should find that Sound, video and game controllers is already opened.
Right click once on the explanation point, a window will open & Left click once on “update driver”. Windows hardware wizard will open. Click No, not at this time. Than click the next button. Another window will open. Left click once in the circle next to, Install from a list or specific location (Advanced). Than click the next button. Again, another window will open. We want to remove the green check mark next to, Search removable media (floppy, CD-ROM...). Left click on the green check mark once & it will go away.
Click the browse button, click the + sign next to my computer. Find the “Sound” folder on your desktop. Open it, high-light by left clicking on it once the correct folder, 32bit or 64bit. Than click OK.
Windows will search within the folder & install the correct driver software. If by some chance windows does not, repeat the above & open the Sound folder. Find the “AsusSetup.exe” within the correct 32bit or 64bit folder & click on it once. Than click, OK. Windows should install the driver. Close all opened windows & restart your system. Test you audio by running “SoundMax" software within control panel”.
Good luck ravishan12!