In human speech, the production of vowels consists in strengthening some specific areas of the harmonic spectrum, known as formants, by adjusting vocal-tract acoustical resonances with articulators such as tongue, lips, velum, jaw, and larynx. In singing, a compromise is often sought between the frequency of harmonics and resonance frequencies, sometimes at the expense of vowel perception. In some vocal cultures, this link between harmonic frequency and resonance frequency is skilfully adjusted. A melody is generated independently of the tonal melody related to vocal-fold vibrations. This is the case of harmonic singing, overtone singing or Xhoomij, practiced in Central Asia, but also of singing by Xhosa women in South Africa. In this paper, the adjustments between harmonics and formants are explored on a wide range of commercial singing recordings and experimental recordings in laboratory. Three main strategies are described from both acoustical and musical point of view. In a first case, the spectral melody is produced by a play on the first formant (F1). The first harmonic frequency is often kept constant and at low values due to period doubling induced by a ventricular vibration. In a second case, the spectral melody is produced by a play on the second formant (F2), with a higher frequency of the first harmonic. Complex spectral melody can also be developed by a vocal game on the first two formants. In particular, we will illustrate and discuss the cases where the two first formants evolve while remaining in an octave ratio (F2 = 2F1).