Brooklyn: Rachel Martin didn’t quite believe that anybody had figured out the economics of audio. “If anyone claims they know how to do it, they’re lying,” she said. “A couple of people are making money, but they’re not here.” It was the opening session of On Air Fest, a two-day audio festival spread across a couple of trendy hotels in Williamsburg, and Martin, host of the NPR show Wild Card, was on stage with Brian Reed, the host of Question Everything.