when you play a sound, it's a series of individual grains played in succession. imagine each individual grain is a snapshot of the sound - it will play the frequencies of the sound at that given point in time. when you stretch a sound but keep the pitch the same, the reason why it has that metallic resonance type sound (called an audio artifact) is because the sound is trying to not leave space (silence) inbetween the grains. if you stretch a sound waaaaaaaaaay out, you can really hear each individual grain of it.