Don't know what this code does
// play file 004.mp3 in folder advert02 -- switching sound
Serial.write(0x7E);
Serial.write(0x07);
Serial.write(0xA0);
Serial.write(0x30);
Serial.write(0x32);
Serial.write(0x30);
Serial.write(0x30);
Serial.write(0x34);
Serial.write(0x7E);
but I feel it should be put in its own function, lets say playMp3File();
This function should be called from shortKeyPress(). Have you read the comments in my code ?