OK, well I have spent the last hour and a half playing in every conceivable way with my laptop and I can't get the mighty mouse sound to happen. In fact, the quality of sound is the best that I have ever heard from my laptop!!? I ran the computer full throttle, maxed the ram and caused massive pageouts, ran almost every major app I own at the same time, ran it while plugged in and fully charged, ran it while unplugged, ran it while plugged in and charging, ran it after a logout, ran it using different electrical outlets from around the house etc.
I have almost completely zero hiss or electrical noise when scrolling from the headphones (grado sr80i, bose triport, bang & olufsen adjustable earphones, apple ipod standard isssue) or laptop speakers. I haven't used my cans with the laptop since the last 10.x.x update so maybe that has something to do with it?
I can't seem to remember if the mighty mouse (MM) scroll type sound was always active on scrolling or if it was intermittent but it is sure not on now. I can, however, hear lots of noise coming from under the keyboard and scrolling does effect that but it is not the same as the mighty mouse sound that I used to hear. (btw, I hear three noises under the keyboard. One noise is always there and sounds almost like a scratchy morse code, the second noise happens only from time to time and sounds like what I think is the hard drive mechanism over the platter?...almost like a series and tiny klink sounds followed by larger klink and then its done, and the third is much more rare and sounds high pitch like a whine, it sounds almost like the first sound but very high and constant for about 3-5 seconds before it drops completely.
I am beginning to wonder if I have ever even heard the MM sound with my headphones on while scrolling or if it was from the same area under the keyboard like the other noises! I'm at least 80% sure that I have though.
Also, I do have pretty bad allergies now here in North Carolina so that may have something to do with the great sound quality I am hearing but I am able to hear the minor noises from under the keyboard from across my den (~20 feet) so I think it shouldn't matter too much. I'll keep at it.