This was going to be for Christmas, but then it turned out more complicated and difficult than expected, and anyway it isn’t particularly Christmassy.
Notes on pitch shifting: the shalalala voices are all pitch shifted, even though I can easily sing the bottom line at pitch. The idea being to make them gel better, since they have all been similarly abused. That worked, except the bottom voice still claimed more attention, presumably because it’s firmly in my natural range and sounds some kind of better. To counteract that without making the top voice too loud, I added another track with the original un-shifted audio put through a high-pass filter cutting out the musical frequencies.
The top two pitch-shifted lines sound remarkably similar to my natural voice if I sing that high, but less strained. Also, I can’t sing that high for that long.