George hears a harsh clanging and ringing. Imagine you were surrounded by 3 fire trucks blaring their sirens, a toddler on espresso with a full drum kit, and a train trying to start and stop on rusty tracks.
Simply put, yes, this sound stops George from showing intelligence. However, the reason for this is important to know. The sound keeps George from maintaining his level of concentration or focus. As a result, he is unable to complete a thought - it is not that George is unintelligent, it is that this sound is stopping him from communicating his intelligence. You could compare it to modern ADHD, which keeps people diagnosed with it from maintaining their concentration as well. Sadly, many people with ADHD are accused of being unintelligent because of their disability, but as in George's case, it is clear that this disability affects communication, not intelligence.
George hears a variety of sounds on his "mental handicap radio." (This is described as small, something like a hearing aid.) In general, they are described as "sharp," though I would call them more harsh or disruptive. One is described as being like a hammer smashing a glass bottle. Another late in the story is like a rivet gun. These sounds are loud and brutal.
Yes, they definitely keep George from showing intelligence, largely because they keep George's mind from functioning as well as it should. In fact, you can be sure that George is intelligent precisely because the sounds are so loud, disruptive, and frequent. Three times a minute--every 20 seconds--one of these noises blasts apart his train of thought. They are all different, so he can't even get used to the sounds and block them out.
Cruel, but functional.