When you
speak, you are hearing most of your
voice via the
vibrations travelling through your head, not sound travelling to your
ears. The sound waves that make up your voice are travelling trough a lot of
bone,
meat, and
sinuses before they make it to your
auditory equipment. Hence, it changes a bit, that's why every voice has a
unique version that only its owner can hear.
Source:a Popular Science I glanced at years ago