In episode 99, the Chimera Ant bat says, "Water significantly weakens the power of sound!"
But water is a good conductor of sound. Why does she say this?
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She probably says this because water weakens the effect of the technique, secret sound, that Bat.
Bat enhances her voice and produces a powerful sound wave infused with her nen that confuses and disorientates her enemies leaving them without guard for some seconds
In this scene, Gon rips a bit of his trouser pocket before making them damp with saliva and stuffing them in his ears, effectively making earplugs for himself.
Practically speaking, fabric itself doesn't make good earplugs, but by dampening it, it works quite well.
If Bat's sound waves cannot be heard as well, it won't have the same effect. Therefore it is weaker.
It's important to note that the original phrase was: 水は音波のエネルギーを著しく弱める
Which can be translated, taking in context, to: Water significantly weakens the energy of the sound wave.
Therefore it could be her referring to how water is weakening the effect of her technique, instead of sound in general.
Because water is more dense than the air, the sound does travel faster through water, i think about 4 times faster than air! but because water is dense it also needs more energy to moves the water particles ! which is why you can hear a faint sound in air but in water it doesn't even start, so you can't hear anything
in the end what you read in the link you mentioned is a talk about sound speed, which is indeed faster in water, and even faster in metal ! but there are variables such as initial energy needed for moving the particles, so you can hear
This site too, explains about sound travel under water