This is a hotly-debated topic. Very hotly debated. There is no definitive answer to it, and you will hear a wide variety of opinions -- so much so that I am always slightly hesitant to re-open the pandora's box of the discussion.

Yes, I have heard cables that improve speaker sound. I have used BetterCables speaker cables, and find them to be pretty good. They are a little expensive, but not unreasonably so -- and they also have a 6-month return policy if you decide they aren't worth your money.

Here's my experience, for what it's worth: I currently use TMC speaker cables to bi-wire my center and to bi-wire my mains along with a pair of Transparent MusicWave Super cables (TMC on high, Transparent on low). When I got my Paradigm Reference speakers almost a year ago, I bi-wired the Studio 60's and Studio CC with Home Depot 12ga wire. My father-in-law gave my wife a pair of older Transparent MusicWave Supers, which I used in place of the Home Depot wire for the Studio 60's. Now, those are very expensive cables (when you have cables that could potentially cost more than the speakers they are connected to, things may be getting a little out of line...), but they didn't cost me anything, which means there's no need to justify the purchase to myself. The difference was amazing, impossible to miss. I wasn't able to bi-wire the Studio 60's until I got the TMC Gold Label speaker cable, because I can't afford a second set of Transparents and nothing else I tried (some older, very large Monster Cable and the BetterCables speaker cables) could keep up -- it sounded absolutely comical.

I do agree with many people who say that a lot of the "high-end" cable market is obscenely overpriced, but my experience tells me that good speaker cables can make a difference in what you hear.

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Gonk