Im not sure what soundcard I have I think its built in to my motherboard which is the asus p5v-vm dh and I didn't get an adapter with my graphics card.

It seems to me that the only way to test if it IS my TV that is causing the problem is to pay the £14 for the cable which id probably inevitably need to do anyway. Am I right?

not necessarily, ask some of your friends if they have a dvi cable that you could borrow

EDIT:they should have one if they have a new-ish lcd

I dont know anyone with a cable 5 metres long which is what I need as I can't move my pc or tv.

I mean, just to test, then get a bigger cablr if it works, surely you should be able to move the pc for a few minutes just to check the quality, if not i dont know what to do because i wouldnt recommend buying another cable when it may not fix any problems