Siopyn

Siopyn was an artificial gas that most sound waves could not travel through. Some law enforcement agencies and bounty hunters used siopyn grenades (often referred to in police slang as "deafeners") to mask the sound of their weapons and movements when clearing buildings.

Siopyn Incident
In 14 BBY, anti-Imperial terrorists on Hylobar - a mining colony in the Mid Rim and the headquarters of the galaxy's leading siopyn corporation - released the contents of siopyn factories across the planet, drowning several population centers in the deafening gas in what the Hylobarians came to call the "Silence Plague"; while victims of the "attacks" could breath normally in the invisible siopyn clouds, there was no transmissible sound in the affected regions until the gas started clearing up with the help of foreign scientists nearly a month later - exposure to thd gas also weakened the immune systems of children, heightening their risk of bacterial infections.