If, as the song goes, video killed the radio star, then homemade YouTube heroes like Tay Zonday have put a hit out on traditional advertising.In April, Mr. Zonday became an Internet phenomenon after he posted a no-frills video for the song “Chocolate Rain” on YouTube featuring his earnest delivery and his deep voice.Two weeks ago, Mr. Zonday, a 25-year-old graduate student in American whose real name is Adam Bahner, posted a follow-up called “Cherry Chocolate Rain.” But in this case, the flashie...