5 Answers

It's because the air is removed

Try this it will blow your mind. Put ice cubes in a dish of cold chilled water then boil in the microwave. The chilled water will boil and the ice cubes will remain frozen. It's because the make up of flowing liquid and solid liquid, also hot or cold liquid is completely different. Hot water produces steam which when poured has to escape through the other water being poured, thus a different sound

Because boiling the water changes the oxygen level in the water

When the heat removes the air [O2] The density changes.

Perhaps as water is heated to boiling point its specific gravity becomes lighter due to the increase in temperature, try pouring it into a cup without a teabag in, as this will also cushion the sound, how many cold cups of tea have you made, test using a teabag and cold water! Stupid