When an audience is so angry or disapproving that they make a hissing sound, they sibilate. Little kids who are missing their front teeth often sibilate too.
You sibilate to some degree every time you pronounce the letter s — the hissing sound itself is called sibilance, or described as sibilant. When your friend tries to catch your attention in the library by saying, "Psst!" she sibilates. And the librarian sibilates too, when he angrily shushes her with a loud "Shhh!" Sibilate comes from the Latin sibilare, "to hiss or whistle."