Each and every earbud makers have a different way in which they capture sound. Some sense the vibration in the jawbone as a person talks. But generally, the idea used is to fit more than just one speaker to the earbuds. With a little engineering which includes the measuring of the time of the travelling speech. The human voice is filtered through the ambience sound. This is done with the help of the frequencies difference in the sound and the spoken voice of a human.