James Levine is amused that his health has suddenly become an issue. “`It seems to be a mystery to everybody,’ he said, how a tremor that’s affected his left arm for more than 10 years suddenly became so newsworthy. Through this `crisis,’ Levine – who’s been artistic chief of the Metropolitan Opera for nearly three decades – has been busy conducting Wagner’s four-night, 20-hour trilogy of operas known as the `Ring’ cycle.”