Pin 18 (typically) handles the "Standby" or "Mute" function. If your circuit isn't producing sound, check that the voltage on this pin is correct; otherwise, the IC remains in "protection mode."
Grounding and layout
Uses a split power supply to handle "spurious shock noise" with a built-in muting circuit. Power Supply Switching: The Class H nature requires two sets of supply rails ( ) to switch voltages efficiently. Protection Circuits: stk413-430 circuit diagram