mySerial.println("a");
This line of code is sending the character 'a' and then a carriage return ('\r') and new line ('\n') character.
What do you think happens with the buzzer when the carriage return and new line characters are read by the receiving code.
if (mySerial.read() == 'a')
{
sound = 2000;
}
else
{
digitalWrite(buzzer, LOW);
}
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