HELP! Changing the melody using buttons while it's playing

[J-M-L]

your playMelody() function does not return until all the notes have been played (see the for() loop?)

@Kamikaza

void playMelody(int melody[], int notes) {
    for (int thisNote = 0; thisNote < notes * 2; thisNote = thisNote + 2) {
        divider = melody[thisNote + 1];
        if (divider > 0) {
            noteDuration = (wholenote) / divider;
        } else if (divider < 0) {
            noteDuration = (wholenote) / abs(divider);
            noteDuration *= 1.5;
        }
        tone(buzzer, melody[thisNote], noteDuration * 0.9);
        delay(noteDuration);
        noTone(buzzer);
    }
}

so you need to break that into chunks and non blocking code using millis() for example

For extra information and examples look at

• Using millis() for timing. A beginners guide
• Several things at the same time
• Flashing multiple LEDs at the same time

you might benefit from studying state machines. Here is a small introduction to the topic: Yet another Finite State Machine introduction