http://www.themodernvocalist.com/profiles/blogs/can-a-stressed-singer-learn
Stress tenses certain parts of the body, such that your vocal apparatus is misshapened, and singing becomes so much more difficult. Being calm is easier to move into the next emotion in the musical interpretation. But as any stressed out singer (any most major pros were at one time starving-stressed) can tell you, it is entirely possible to sing when stressed.
Stress can even help certain types of singing. For example, if you’re sad, and emote sadness, singing sad songs can sound better. Of course, singing happy songs is more difficult.
As for your instructor– she got it partly right.
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