Need to make an action just once when condition is reached
A possible solution to your problem can be the following:

#include &lt;PCM.h&gt;
const unsigned char sample[] PROGMEM = { //Here goes the sample
};
int low = 0;
int mid = 0;
int high = 0;
bool first = true;
bool second = true;
bool third = true;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);

}

void loop() {
  // put your main code here, to run repeatedly:
  low = analogRead(A0);
  mid = analogRead(A1);
  high = analogRead(A2);
  Serial.print("nivel agua: ");

  if (high &gt; 50 &amp;&amp; first){
    Serial.print("Alto");
    startPlayback(sample, sizeof(sample));
    first = false;
  }
  else if (mid &gt;50 &amp;&amp; second){
    Serial.print("Medio");
    startPlayback(sample, sizeof(sample));
    second = false;
  }
  else if (low &gt; 50 &amp;&amp; third){
    Serial.print("Bajo");
    startPlayback(sample, sizeof(sample));
    third = false;
  }
  delay(100);


In this way the sound for each level will be played only once. I hope this is what you were looking for.