Computer does't recognize audio devices

hi,
I have a weird problem:
I just bought a new computer with NVIDIA GTX-750Ti graphics card and windows 7 ultimate.
I'm now trying to install my headphones, and the computer just doesn't reconize that I connected my headphones - or any other audio devices.
I think the problem might be my new screen: it have amplifiers and I connected it to the computer with HDMI cable that trasnport audio too. that caused a collision between the two audio devices.
I'm not an english speaker so forgive my english mistakes ;)
Thanks a lot!!!