I have heard differences between cables ranging from the almost nonexistent to the obvious. The only physical difference between AudioQuest Clear and AudioQuest Sterling speaker cables is the conductor: high-purity copper in the former, high-purity silver in the latter. Yet the improvement in low-frequency definition offered by Sterling is so large that it might almost be audible in a blind test :-).
In my view, RLC advocates take too simplistic a view of audio-signal transmission—there are other factors that affect a cable's electrical performance. The first three I list are properties of the cable itself; the second three are properties of the system in which the cable is used. (I'm sure there are more.) Yes, these are all secondary effects, but if R, L, and C are the same (not that they often are), what else is left?
• A decade ago, Malcolm Omar Hawksford of the University of Essex in the UK derived from first principles the conclusion that there is an optimum conductor diameter for audio-signal transmission (see "The Essex Echo," Hi-Fi News & Record Review, August 1985, [reprinted in revised form in Stereophile, Vol.18 No.10, October 1995]). I've yet to see a refutation or even any discussion of Professor Hawksford's conclusions, but perhaps coincidentally, many of the "audiophile" cables tend to use conductors of this diameter.
• As an electrical field can't exist in a conductor, and as the audio signal represents a varying electrical field, it must travel in the dielectric medium surrounding the conductor. The primary effect of the dielectric is to reduce the speed of transmission, though this remains a significant fraction of the velocity of light. But as a cable is nothing more than a stretched-out parallel-plate capacitor, and the adverse effects at audio frequencies of using a sub-optimum dielectric in capacitors have been well-documented, isn't it reasonable to suppose that the dielectric can't be ignored in audio cable design? Certainly the best-regarded audio cables use "well-behaved" dielectrics.