Hi,
Is the onboard Audio Enabled in the BIOS?
Download Drive Sweeper http://www.guru3d.com/category/driversweeper/ And remove all the sound card drivers and restart and then install these sound drivers - http://www.asus.com/Motherboards/AMD_AM3/M4A88TDV_EVOUSB3/#download
Once installed restart again and then also make sure your Audio Output device is also working. And then plug in the headphone/speaker's 3.5mm jack in the green port on the motherboard back panel.
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Plug in the audio output device in the green port.
Here try this source - http://majorgeeks.com/Driver_Sweeper_d6171.html
First try this first.
And let others also suggest some things which you can try, if none of them work then there would be a problem with the onboard sound card, then it would be time to RMA the motherboard.
Are you using the front panel connection or the rear one on the motherboard? If it's the front one, try using the rear. You might have plugged the front panel header into the wrong thing, or not plugged in at all. Test your speakers/headphones on another system as well. Make sure they are plugged into the green port as well.