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Here are some links to some of the best free web articles dedicated to helping you learn to sing that know of. I could type all night and well into tomorrow and not be able to put it as well as XXXXX XXXXXm.
1) Singing Power
2) Extending Your Vocal Range
3) Vocal Flexibility and Accuracy
4) How to Develop Your Vibrato
5) Sustaining Your Voice When Singing
6) Developing a Smooth Voice 7) Your Personal Singing Guide