Most Helpful Guy
I believe it all has to do with your heart-rate.
Like when you go for a jog... the song slows down the harder your body is working. Then say you wake up to music you left on in the middle of the night... the song sounds much faster since your heart-rate is slowed and your body is in a relaxed state.
Just my theory. I haven't actually looked into it to find out for sure...1