Something that sounds deep and soft, especially someone's voice, can be called low-pitched. It's polite to speak in a low-pitched voice before your houseguests have woken up in the morning.
People who sing in low registers — altos and basses, for example — have low-pitched voices, and a big dog might have a low-pitched bark, especially compared to your toy poodle's yap. This definition of low-pitched comes from the sense of pitch that means "the highness or lowness of a sound's tone." Another way to use this adjective is to mean "at a low slant or slight slope," from the pitch meaning "steepness of slope."