“When we started out we knew virtually nothing about copyright,” says Nico Perez, co-founder of Mixcloud, a four-year-old online music service that already boasts more than 3 million users a month. A kind of Internet radio, Mixcloud is free to consumers, allowing users to stream but not download content (DJ mixes, for example) uploaded by other users. It’s social media meets music provider: a YouTube for radio.