The MIX Voice is not just one Vocal Register, but a variety of blended coordinations. The Mix mostly boils down to the STRENGTH of the ThyroArytenoid Muscle MIXED with the LENGTH created by the CricoThyroid Muscle. This means power without volume. This means Head Resonance without Head Voice. This means high notes that feel just as easy as middle and low notes. To improve your Mix Voice, try carrying some of the strength of Chest Voice up higher in your range - yet without increasing the air, without adding volume, and without opening up the mouth wider. It's very tempting to cheat with breath, volume, and open embouchures. But, if you stay true to the mechanics, you'll find your Mix coordinations developing - STRONG and LONG!