SoundCloud is the ubiquitous wild child of the digital music world. Without paying artists or record companies, it lets people freely upload and stream any kind of audio, and musicians have embraced it as a way to share their hottest work with fans. Lorde, the teenage pop star from New Zealand, rose to fame after posting her song “Royals” there. Even in a market saturated with competitors like iTunes and Pandora, the six-year-old SoundCloud has managed to reach a huge scale with a catalog of unusual, often exclusive content. According to the company, about 175 million people listen to music on its platform each month — more than four times Spotify’s global audience. “We have listeners in every single country in the world — and in space,” said Alex Ljung, SoundCloud’s chief executive, referring to recordings of the International Space Station posted by a Canadian astronaut.