GRUENHEIDE, Germany—The first cars to emerge from Tesla’s new Berlin factory should roll off the production line as early as next month, CEO Elon Musk said at the site of the plant on Saturday, but added that volume production would take much longer to achieve. Musk hopes to get the green light in coming weeks to start production at the site. The latest consultation on public concerns towards the site closes on Oct. 14, after which the Environment Ministry will make a decision. “Starting production is nice, but volume production is the hard part,” Musk told a cheering audience at a festival at the plant site, many of whom livestreamed the speech on social media. “It will take longer to reach volume production than it took to build the factory.” He said volume production would amount to 5,000 or “hopefully 10,000” vehicles per day, and battery cells would be made …