I’m guessing an impedance mismatch, if the amp input doesn’t load the headphone output enough the output might go a bit nuts [oscillation, feedback etc]. Turn the volume to max on the PC and Min on the amp and try bringing the volume up on the amp. Better signal/noise ratio?
Have a really good dig around in the Sound control panel: Try the Properties and Configure buttons, and then mess with the settings a bit [try to remember the original ones, natch].
Is there another headphone output [on the front, for example]
If you have an HDMI input and cable handy – try that.
If you could list the make and model of the machine that would be dandy, best of all if you could find the model of the sound ‘card’ that would also probably help:
[Through the search box:
Microsoft Windows users can determine their sound card by following the steps below.
Click the Start Button in the lower left-hand corner of your screen; or navigate to the Start Screen (Windows 8).
Locate the Search Box in the lower left-hand corner of the window that appears; Windows 8 users may just start typing.
Type dxdiag and press Enter.
You sound card is listed under one (usually the last) of the sound tabs at the top of the window.]
I’m a major audio geek, I feel your pain. Don’t worry, we’ll figure it out soon.
Best of luck I’m hitting the sack.