Go to the AsusTeK webpage and download & install the latest audio driver
VIA Audio Driver V6.0.1.7400. While you are there bookmark the page and get the others Bios & drivers updates for your board and Windows 7 OS.
Also if you have any front audio jacks, where you plug in headphones for example, unplug all those and make sure your speakers are plugged in to the proper speaker out jack in the rear (it should be green).
If you still can't hear things properly after those things then try
Microsoft Fix-it for audio problems.