As in other instruments, the sound of the voice is the result of amplified vibrations.
 
In the guitar the sound produced by the vibrations of the strings resonates in the sound box.

The vibrations of the vocal cords resonate in the chest (lower register) or in the head (upper register).

Our body has some parts that participate in the resonating system amplifying the sound of our voice:
the throat (farynx), the head (head bones, oral and nasal cavities), the chest, the bones...
When we sing, all our body can be a sound box.