unsigned long previousMicros;
const unsigned long interval = 12500;
bool active = false;

const int ledPin = 13;
const int outputPin = 2;

void setup() 
{
  pinMode( ledPin, OUTPUT);
  pinMode( outputPin, OUTPUT);  // not required for 'tone()'
}

void loop() 
{
  unsigned long currentMicros = micros();    // 4 microseconds accuracy on Arduino Uno

  if( currentMicros - previousMicros >= interval)
  {
    previousMicros += interval;      // special code to keep in sync with time

    if( active)
    {
      digitalWrite( ledPin, LOW);
      noTone( outputPin);
    }
    else
    {
      digitalWrite( ledPin, HIGH);
      tone( outputPin, 8000);
    }
    active = !active;
  }
}