Music fans who want to mix games and social networking while listening to songs on the Internet now have a site called “Loudcrowd,” created in part by developers behind “Rock Band” and “Guitar Hero.” The site is to make its public debut Tuesday at the South By Southwest Festival, an annual gathering for music, film and new media fans and insiders in Austin, Texas. Dan Ogles, one of its cofounders, hails from Harmonix — the video game developer behind “Rock Band” as well as “Guitar Hero” before that game’s development was transferred to another studio.