Tone of your voice and diction can be matter of inborn attributes, but still can be exercised and developed, just as any other skill. Here are set of different exercises and techniques that can help you to improve voice and pronunciation.
1. Stretching and yawning. Relax. Exercising posture: standing with both feet on floor, shoulders and arms are relaxed.
2. Deep breathing with diaphragm and abdominal muscles. Inhale through half closed nose (one nostril closed, other is half closed with fingers), exhale on mouth with s-s-s-s sound.
3. Extended, single exhale of "a" voice. Shape the "a" vocal with abdominal pressure on lungs and try to resonate in throat and mouth with least effort.
4. Create and resonate "om" vibration and try to tune the vibration so you can feel it from lips to the whole body.
5. Pronounce numbers in a fast, bursting manner, while putting accent on emphasized syllable.
6. "Ha" exercise – Inhale short and silent. Exhale with "ha" sound, loud from your abdomen.
7. Massage of your larynx, with gentle circular moves from both sides, slightly pushing your larynx downwards. Push your chest forward and touch your sternum and try to feel vibrations while you speak. Enhance vibrating sensation by quick tapping on your sternum.
8. Pronounce "yeeee" and try to feel vibration between your tongue and palate.
Alternate sound "yeeee" and other words and try to feel the same sensation of vibration on words.
9. Pronounce words and numbers with blocked nose (with fingers), while trying to avoid nasal accent and vibrations in your nose. Alternate pronunciation with and without nose being blocked.
10. Count from one to a hundred, by whispering quick and vigorously. Use mouth and face muscles extensively. Whisper only while exhaling.
11. Speak loud with strong diction, text can be totally improvised. Pay attention on abdominal contraction.
12. Argue loudly and exaggerate with excessive gestures. Text is improvised.
Exercises should be performed on daily basis for the long period.