Riding high on the growing consumer demand for podcasts, a startup called Castbox this morning announced the close of $13.5 million in Series B funding for its technology-fueled podcast app. The round was led by SIG China, and includes participation from existing investors IDG Capital, Qiming Venture Partners, and GSR Ventures. To date, Castbox has raised $29.5 million. While there are a number of podcast applications on the market today, what makes Castbox interesting is the proprietary technology it has under the hood.