This web page shows the phenomenon of the Speech to Song illusion. When we speak, we are actually speaking with a melody. Our brains and ears don't pick this out though, in everyday speech. However, if you isolate a particular word or phrase in a sentence and repeat it, because if this repetition your brain picks out a melody that can even be sung and repeated by others. There are example sound bytes and a video that demonstrates this effect. There are also charts to explain exactly when this happens.