Torres is the Spanish luthier who set the pattern for classical guitar construction. At the time, there was a lot of debate about the importance of the construction of the sides and back. To prove his point about the dominance of the top/bracing, he made a guitar with paper-mache sides and back. It was said to have sounded very good.
Back and side construction have an effect on the sound of an acoustic, but, it's probably subtler. Most of the string energy and the largest displacements are in the guitar top. The body of the guitar is a resonator and the top is driving the volume of air. The primary job of the sides/back is to support the top, define the shape of the cavity, and help project the sound out of it.
The curved shape of the sides make them very stiff, compared to the top, and they vibrate much less. The backs are also stiffened with bracing and arching. The density, stiffness, damping, and mass of the sides and back affect their response... how brightly they reflect sound, what their own natural frequencies are and how that reinforces or detracts from the guitar's overall sound.
The material for guitar (and violins/cellos/etc) tops has converged on really light, strong, straight grained tone woods. Even on expensive instruments, sides and backs are made of other species of wood which are frequently chosen as much for their appearance, as their tonal properties.
BTW, I'm new to this forum. I recently stumbled onto the site. It's really cool. Some amazing and inspiring stuff is being done here.