How it would work...

"You say that the Sound type wouldn't work and that most Pokemon can make sounds? Well, I'm pretty sure most if not all Pokemon use Normal-type attacks, not to mention are based off of 'normal' animals, but you don't see 90% of the Pokemon out there being of the Normal type.

Makuhita can learn Whirlwind, so should it be a Flying type? And everyone and their mama can learn Aerial Ace... plus all the non-Fire pokemon that can learn Fire TMs, etc.

Sound type could DEFINITELY work. And here's how (including Wind and Light... for fun):

Offensive
-Strong Against: Bug, Dragon, Flying, Fighting, Normal
-Weak Against: Dark, Psychic [Immune], Steel, Ghost [Immune], Water, (Wind, Light)

Defensive
-Weak to: Psychic, Dark, (Wind, Light)
-Resistant to: Fighting [Immune], Normal

While most every Pokemon can use sound as an attack, only Sound type pokemon would 'specialize' in it. It's identical to why Fighting types exist- ALL Pokemon fight, but only fighting types 'specialize' in it. Fighting, Dark ('Evil' in Japan... which is a CHARACTER trait) or normal are as much of a characteristic as sound.

Now to go through the reasons...

Offensive: Sound attacks would be super-effective against all 5 of the "animal-like" (<- term used loosely) types (Bug, Dragon, Flying, Fighting, and Normal). This is because real-life animals/insects/beings have a weakness for high-pitched/cacaphonous (sp?) sounds. For instance, my house has things plugged into the wall that emit ultrasonic sound waves to keep away rats, spiders, and ants. Psychic, Dark, and Light types would be special types that aren't really affected by sound types (in fact, Psychics are immune as their mind waves would block the sound [or something... hard to explain. sorry, it just makes sense to me]). Sound passes through water fairly easily so they would be resistant. And now Steel types.... steel types IMO are the collective name for the pokemon based on metals, and MOST metals block most sound waves fairly well. In fact, some metals carry sound waves as vibrations. So they're resistant as well. Ghost types are transparent (supposedly), so Sound attacks wouldn't even touch them. And the Wind disrupts and even blocks sound waves if strong enough.

Defensive: Psychic, Dark, and Light types, being 'special', would realistically be the only types to super-effectively 'hit' sound (considering real sound is intangible). And Wind types, being able to disrupt sound waves, should be super-effective as well. In the same sense that sound is intangible, Fighting attacks would never be able to hit Sound types. And normal attacks wouldn't fare much better. [Steel is not included as it might make Sound types unbalanced]."