Your voice depends on muscles, so like any muscle it needs exercise to be strong.
We had voice classes in school and much of it was about learning to relax the next, and the muscles around it.
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One exercise that I have been taught recently that has been really helpful is a slow sun-salutation like warm up. Stand parallel, feet shoulder width apart.
Inhale, raise your arms.
Exhale – slowly bend over to touch your toes (or however much you can do so). What’s important here is that you feel the connection in your airway. Keep your throat and mouth open as breathe out with your mouth, and let your hands travel a large arc in front of you on the way down.
Inhale- roll up back to the hands-up position (if it’s helpful bend your knees and curl your spine up);
Exhale – Bend over again, and then in the same motion, gently drop to all fours.
Inhale- leading with the head, come to a cobra pose
Exhale – Bring your butt to your heel into child’s pose
Inhale; exhale – rest
Inhale – keeping head down, return to the bent over pose.
Inhale – roll up bringing your hands up
Exhale – bring your hands down to a gentle “prayer” (palms together) in front of your chest.
Repeat this cycle, keeping your movements as slow as you can make it. Make sure you coordinate the movement with your breathing, and exhale completely. So you have to push the air gently up from your lungs.
Then, on the third cycle, on the exhale/cobra vocalise a “WAWWWW” (rhymes with “all”), full open throat. Just let the voice come out and fill the room, keep the throat relaxed and don’t push it. It’ll get stronger. Rather, pay attention to the resonators / the feeling of vibration on top of your head, and back. When you are out of breath, continue the cycle, as many times as you like.
This exercise is great for building a strong voice. Recently I’ve been teaching, and it’s made a huge difference in being able to project for long periods of time. It’ll also make your voice sound more resonant and full.
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If people are interested I’ll post other voice exercises; it seems like what you want to do is also work on your range, and practice imitating as many voices as you can.
Last note to do with voiceovers – the first time you come into contact with whoever is hiring you (whether on the phone or face to face), they will be listening to the quality of your voice. So don’t just think “it’s just a phone call to set up a meeting”, you’re already making an impression. It’s true with any job interview but in this case you should be particularly attentive to the impression your voice is making.