There are two elements involved: 1) the surface that acts as a diaphragm--which is eithe the plywood or the peg board--that pushes air based on the diaphragm's ability to flex; and 2) the reflective surfaces that receive the sound waves from the air and either reflect or absorb them. Plywood is tightly bonded wood particles, and the "ply" indicates its pliability or flexibility to act fairly well as a diaphragm. Peg board is made of smaller particles pressed and bonded together. Although the multiple holes will reduce the amount of air pushed slightly, the material itself is more deadened than plywood and somewhat less likely to conduct noise.
Once sound has spread out from a source, acoustic tiles--also made of "deadened" material--have the holes to deflect air pressure and reduce reflectivity of the sound waves. (My father was an architectural engineer who had installed acoustic tiles in a school ceiling in the early 1960s. He explained to the janitor that the holes "absorbed the sound" to make things quieter. The janitor's face lit up in rage: "How the **** am I going to clean the sound out of all those ****ed holes every day?" That's a true story.)
