"music that played through the earphones was supposed to change in pace and volume as the user's heart raced during a workout. Users would be able to control how much ambient sound the earphones let in -- listeners would be able to block out the outside world, or set the earphones to let them hear oncoming traffic while biking or walking."
I'm not a scientist or music engineer, but that "sounds" impossible. Pun intended. Critical thinking skills will tell you that your heart beat cannot change the pace or volume of sound waves. Especially if the point of origin of the music is outside of your body.