How to connect one pin to ground when an another analog pin is triggered
Can you please connect the sound board in the usual way, Arduino ground to sound board ground , Arduino pin 2 to sound board play pin 1 and modify the output for inverted logic (LOW = play and HIGH = stop  set as input = stop)? 

Obviously it plays all the time because the soundboard play pin is always connected to soundboard ground pin. 

This is your first setup:







simulate this circuit &ndash; Schematic created using CircuitLab

First option:







simulate this circuit

Arduino uses 0-5V I/O levels , the soundboard uses 0-3.3V that's why  you should use only  pinMode(SOUND_PIN, INPUT); disconnecting the sound play pin instead digitalWrite(SOUND_PIN, HIGH) that pulls the pin to 5V when you want to release the pin.

To pull the pin to GND use digitalWrite(SOUND_PIN, LOW);pinMode(SOUND_PIN, OUTPUT); 

See the folowing example:

const int SOUND_PIN = 2; //Trigger Sound Board
void setup() {
  pinMode(LED_PIN, OUTPUT);
  pinMode(SOUND_PIN, INPUT);// high impedance, sound pin disconnected
  digitalWrite(SOUND_PIN, LOW);//make sure the pin will be low when using it
  //this last instruction has no effect yet as long as the pin is configured as input
}

void loop() {
  int piezoADC = analogRead(PIEZO_PIN);
  float piezoV = piezoADC / 1023.0;
  if (piezoV &gt; 0.1){
    playSound();
    delay(5000);
    stopSound();
  } 
}

void playSound(){
  digitalWrite(SOUND_PIN, LOW);//make sure the pin will be low
  pinMode(SOUND_PIN, OUTPUT);// set as output to drive the sound pin to gnd
}

void stopSound(){
  pinMode(SOUND_PIN, INPUT);// high impedance, sound pin disconnected
}


You can use the resistor R1 for the development stage to protect the soundboard input from software errors. 

Other options:

Using an optocoupler as you can see in this link that I found from this similar older question. The S2 must go to Soundboard ground ans S1 to PIN 1

Using a relay, here is an example but you can find plenty on the web