enum Mode {
  fixed,
  changing,
  music
};

Mode currentMode = fixed;

void next(Mode &in) {
  switch (in) {
    case fixed: in =  changing; break;
    case changing: in = music; break;
    case music : in =  fixed;
  }
}

void readable(Mode in) {
  switch (in) {
    case fixed: Serial.println(F("changing")); break;
    case changing: Serial.println(F("music")); break;
    case music : Serial.println(F("fixed"));
  }
}

void setup() {
  Serial.begin(115200);
  Serial.println(F("n for next"));
}

void loop() {
  if (Serial.read() == 'n') {
    next(currentMode);
    readable(currentMode);
  }
}