by slowing down the song you are changing the hz notes ?
a 60 hz tone may become a 40 hz tone if you slow the song enough
I guess is how it works ? also possible clipping ?
did you check the waves as you were recording them
when I use my audacity program I always have to mess with the settings for every song recorded so that I can record it at the highest level possible with NO clipped spots in the song. if when recording I watch a peak go above the limit I delete the whole dealy and start over again with a lower setting. its a pain in the azz
but nobody likes clipped stuff