GarageBand Voice
The Voice sound is sample based. It emulates a mixed choir.
Volume slider: Sets the overall volume level of the instrument.
Filter Cutoff slider: Allows more (high) or less (low) of the sound through, making it brighter or damping it.
Attack slider: Makes the sound start more slowly or quickly. A fast setting makes it sound like striking a piano key, whereas a slow setting makes it sound like bowing a violin string.
Release slider: Determines the time it takes for notes to fade out after you let go of the keys on your keyboard.