I haven't experienced this, Frog, but when we moved into the house fifteen years ago I checked every outlet for correct polarity. I've had no reason to change anything other than to add a few outlets which have always been red=red and then checked again with the outlet tester just to be certain. Unfortunately, none of the outlets are easily accessible to make a swap.

You're talking electrical outlets. I'm talking light switches. There's usually at least one in every room, and they're placed high so easily accessible, and fairly easy to rewire.

Does one outlet in the living room affect the sound if I'm in the other end of the house? Or is proximity involved here?

No, proximity is not involved, from what I've seen so far. If my observation is correct, the wiring of any light switch will affect any audio system. As you know, some people change their electrical outlets to Hubble hospital grade (as I once did). I suspect the light switch wiring though, may have a more important influence on the sound. (But don't quote me on that, as I have never compared the two tweaks directly).

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MP3 player, as in sitting with earphones and listening to it with no connection to your Hi FI gear?

Yes, exactly.

I am full of agreement about the effect you mention as it relates to gear that may be plugged in, not clear as to how 'apart' you mean your listening is from the actual Hi Fi system.

Do you notice different effects in random places when you listen to your MP3?

That would be interesting...

Yes, I think it is interesting. It doesn't matter where I am in the house. The wiring of the light switch affects the sound on the MP3 player, as much as it does any audio system in the house. I classify the effect under a phenomenon I call "advanced polarity". Which itself is part of a larger phenomenon, of which you can probably guess by now.

With all the idiots of the world wiring things, you'd almost think you could identify properly wired rooms you encountered!

No, I'm not that good. I can only identify what I think is properly wired, after opening up the light switch and critically listening to both conditions. Although I don't necessarily feel I've done enough of these experiments to put a 100% stamp on this (hence the post, to see if anyone was willing to help with my research!), I've never not been able to identify a difference in the attempts I've tried; be it at my place or a friend's. But I don't listen to a stereo and say "Hmm.... I think what's wrong with the sound here is the light switch must be wired wrong". However, I have been known, often enough, to properly identify incorrect speaker polarity, without knowing in advance there was a problem with the wiring.

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Maybe I misunderstood something in the original post. First of all under no circumstance should anyone use a dimmer control on any circuit that contains any load other than an incandessent lamp. Ever. Dimmers use either SCRs or TRIACs both of which do terrible things to the shape of a sin wave. Normal AC wiring uses Black and White wires. Red wires usually are reserved for 220 or circuits with two circuit switches. While an AC signal by definition has no polarity, One should not switch the wires to an outlet because many appliances are designed based on the electrical code using the larger left side connector as being tied to neutral and not being hot.

Maybe you misunderstood a few things. We were talking about light switches, not electrical outlets. 110/120v stuff. Sometimes the wiring is black/red, sometimes black/white, and sometimes you can't tell, because at my friend's place where I was installing the dimmer and doing the audio experiment, the insulation is burned up, so both wires are a shade of grey/black. I never talked about using a dimmer on anything but an incandescent fixture (halogen, actually). Although some fluorescent bulbs and fixtures can handle this. Yeah, I'd rather not use a dimmer, for all the corruption it does to the line and whatnot, but I felt it was really necessary in this insatllation.

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Frog-is your sound system on a separate dedicated power line comming from it's own cct. breaker in your power panel ?

It wouldn't matter. As mentioned, the change in sound could be observed on a battery operated mp3 player, as well as electrically operated audio gear.