Ronniekoh,
Unlike power cords, speaker and interconnect cabling can "directly" affect inductive speaker loads, or sensitive gain stages. Both cables are in the audio stream, and can affect frequency response by inductance, resistance, and capacitance. The altering or addition of these passive components may/can be proven audibly (pending their values) instantaneously. A power cord's job is to transfer available voltage/current into the components power supply, thus converted to stable, low ripple DC. This DC will be attained no matter what conductor material is sellected to feed it. Silver or gold conductors will merely grant but a few milivolts difference (over a 6 ft length) into the isolation transformer, over copper. The outlet voltage varies far more at any given moment! Whether being fed by 50 Hz or 60Hz, the power supply converts it into DC, and is un-affected by the passive components mentioned earlier.
Besides all the preceeding footage of non-audiophile Romex wiring, what about all the "acclaimed" aftermarket line conditioners being inserted between the outlet and audio components? Lots of passive/active components in direct contact with the AC stream, thus feeding the exact same audio component. All of which would add way more coloration in comparison to ANY power cord related issues. Oh...and yet ANOTHER power cord! Where's all the "frequency" related questions there? It just doesn't apply to power supplies and cords, with regards to limiting a component's output frequncy. Where's all the concern about the isolation transformer's copper wiring, or core composition? Just think about each and every component's internal soldering, for those bringing up that PC-related topic (includes most power, speaker, and IC cables).
For every personal scenerio you've witnessed/debated someone claiming to hear differenses, I can match with ones who have failed during blind testing, whether in their personal environment, or a dealer's. I've been at this HiFi stuff for three decades now, and been through these discussions/testing before.
My problem is that I lack the discipline with not responding to these forums. Seems no one ever seems to sway the other camp, but in my lengthy career of electronics trouble-shooting and personel training, I've observed all kinds of scenerios where work colleagues report having repaired/diagnosed a problem that wasn't even related, but in fact, they reset or disturbed something else in the process. My explanation/advice to them is to re-verify, sabatage, or replace items they've swapped. More than 70% of the time, they didn't truly fix the problem. That's my advice to you readers. Ignore the label ego and bragging rights, and create engineered methods to reconfirm what you're hearing. That's all...