Sound distorted - both internal and external sound cards - motherboard to blame?

[SexualPotatoes]

So here`s the thing: I just bought a MSI ATX DDR3 2600 LGA 1150 for a new build. I installed Windows 8.1 on it, and it was working fine (it is the computer I am using right now to type this), but the sound is horribly distorted on both the front and back jacks at any volumes.

I then plugged in an external soundcard that I have (M-Audio Fast Track Pro) and I realized that the sound there was ALSO distorted, though considerably less. There is no distortion if the sound is below 70%.

I formatted the computer multiple times. I have a 600W Corsair PSU, a GeForce 760GTX, 8GB Ram and a i5 processor. I checked all the connections and everything seems to be in order.

What could it be?

 

[Karsten75]

Drivers or the speakers you're listening to.

 

[SexualPotatoes]

I updated all drivers, of course, and I tried listening through external sound cards as well as the onboard one. Nothing audio related is working properly, even after formatting and reupdating several times.