We heard Nichelle sing in "Charlie X" and "Conscience of the King." Calling her "enormously talented" as a singer is hyperbolic, I'd say.
I'm not a first-hander on this, but I've read that she dazzled people in her stage shows with extraordinary singing. Thus, what she sang in the two ST episodes is not representative of what she could do with the right songs and a little freedom. In the episodes, Pegasus was hitched to a plow.