The first thing I see is the correct sound driver is not installed. As you installed a new hard drive, although Windows 7 will install a lot of drivers, I would suggest installing all the ASUS supplied drivers, especially the motherboard chipset drivers and RealTek sound driver. The Windows installed HD Audio Codec is only a basic function sound driver.
I have a new system with a Gigabyte Z77 motherboard and even though Windows 7 installed some drivers, I found installing the Gigabyte supplied drivers fixed some problems, including sound.
One other issue, I notice this board has wi-fi. If you are not going to use that, disable it as wi-fi has known to cause some audio clicks and pops issues.