“The actuator arm position influences the sound level, ” explains jedi1983. “In my case I wanted to see the actuators arms moving to the music. Music fidelity was not my goal, so they are free to move.”
Jedi1983 built a wooden sound box and placed it under the hard drives to improve the sound quality, although in hindsight they say “it would have been better to build the box in the shape of a flaring horn.”
Instructions to build your own hard disk speakers are available here.