http://www.amazon.com/Logitech-3-5mm-Jack-Audio-Adapter/dp/B0058P0I2C
This might fix the issue if your 3.5 mm in of your motherboard is faulty.
If that doesn't work invest in a good usb dac,what it does is that it takes audio from your pc by usb and decodes inside it,basically a soundcard but infinitely better.
fiio e7,it is a dac mainly for headphones,but since you use a powered speakers 2.1 i guess it will have no problem.