I have heard music through a variety of USB, speaker wire (14-16 AWG < 6 meters roughly) and RCA cables of varying lengths, brand, and prices. Did not hear a difference all else the same. That said, if there was poor contact I heard a difference: No signal. Same could be said about cables for video.
As far as headphones, I have tried a variety of them, and the differences in sound quality were relatively obvious.
The fact that I heard no difference across different cables and the fact that I see high prices on cables that made no audible difference to me (i.e. Monster at my bro's house) makes me a bit critical of them. Furthermore, I have a feel for how much capacity some cables have, and that makes me even more critical of them... compared to other components of a sound reproduction rig.
I did hear thumping and cracking through some headphone cables when running or when tapping them though: microphonics. But that's about it as far as I can think...
Edited by ultrabike - 9/5/12 at 8:31pm