Clear Channel, the nation’s largest radio station operator and an outdoor billboard company, last year became the biggest leveraged buyout ever in the media business, after it was taken private by Thomas H. Lee Partners and Bain Capital. Now its revenues are plunging and so is its cash flow, making it harder to meet the payments on the billions in debt accumulated in the process of buying out its public investors. If it violates some of its loan agreements, those interest payments rise sharply.