A big HIFI magazine in scandinavia tested a heap load of HDMI cables, all from the cheapest to the most expensive one you can buy, and they came up with a variation on about 2% in picture quality. And the most expensive one, where not the best one.
But something that DID have an effect, where how many times you could twist and bend the HDMI cable itself before it showed up in picture quality... Here, the expensive ones took the lead, after about 4000 bends...
But ofcours... if you fill the cable with a crappy signal, you got crappy quality.
I wouldnt use hdmi just for audio, i think there are better cables out there.