Answer
See below.
Work Step by Step
A guitar relies on a sound board, which is a surface that the strings vibrate against, and set into motion. The sound is made louder because the the vibrating sound board has a large surface, and moves a lot more air than the vibrating strings can move on their own.
When the strings are clamped to a massive workbench, the workbench doesn't move much, and the sound is not as loud.