Play the .mp3s, or use a keyboard or keyboard app to play a note, and major chord, see if the children hear the difference. A chord is multiple notes played together, the note is just one pitch. Have children tell you what they hear. Once they have that, try playing a major chord (happy chord) and minor chord (sad chord) to see if they hear the difference. Children do better at hearing the difference when you start with a major chord, then they h ear the contrast of a minor chord.