under the OEM licencing agreement MS may not authorise the validation of the reinstall.
However saying that I couldnt find one of my retail licences at weekend and ended up using the oem licence that I had on the pc, and despite having a processor, motherboard and graphics card change, it was validated over the net.
As for your sound problem, how bad is the actual hiss. Could it be that the volume is set too high. Having gone from Asus to MSI, I find for some strange reason that I no longer need to have the volume level as high as I did previously.
If that isnt the problem, then I'd uninstall the audio drivers, presumably this will be Realtek as its a gigabyte board, restart machine and then let Windows update install the drivers themselves, and see if that solves the problem, as opposed to installing from the Gigatebyte CD, or alternatively go and download the latest audio driver and install it yourself.
If the fault is still there, then you are going to have to bite the bullet and go for a total windows reinstall.