In the wake of high-profile podcasts such as “Serial” and “Invisibilia,” the once obscure audio storytelling technique has moved to the mainstream.As the selection of podcasts on services like SoundCloud and iTunes continues to expand, the future for the medium looks bright, according to the panelists at the “State of the Podcast 2015” event hosted by Harvard University’s Berkman Center for Internet and Society and HUBweek, co-founded by The Boston Globe.