CTRL: on, off, left or right

Three bands on one pin

The NS4168 reads the voltage on its CTRL pin and sorts it into one of three bands, by its datasheet:

The gaps between the bands are not defined. A voltage has to land inside one.

What sits between the pin and the chip

The header's CTRL pin does not reach the chip directly. It goes through a 22 kΩ resistor. On the chip's side of that resistor, the L/R jumper leads to 10 kΩ and ground.

With the jumper open, as it ships, the 10 kΩ is not connected, so nothing pulls the chip's pin down and it sees the whole 3.3 V: right channel.

With the jumper bridged, the two resistors form a divider: 3.3 × 10 / (22 + 10) = 1.03 V, worked out, which lands in the left band. The same HIGH from the same GPIO now picks the other channel.

Drive it, always

Driven LOW, the chip is off whatever the jumper says. Left unconnected, it has nothing defining it: with the jumper open the chip's pin floats, and with it bridged the 10 kΩ holds it at 0 V. Neither plays.

So CTRL goes to a GPIO, GPIO 18 in this book, and the sketch drives it HIGH before it plays:

pinMode(PIN_AMP_ON, OUTPUT);
digitalWrite(PIN_AMP_ON, HIGH);       // wake the amplifier

Driving it LOW again is the clean way to silence the speaker between sounds.