No company exemplifies the travails of digital music services better than SoundCloud. Founded in 2007, the Berlin company has attracted about 175 million users with 135 million tracks of easy-to-share remixes, podcasts, and user-generated content. SoundCloud is beloved among artists and fans, and a darling of the European startup scene. It’s also facing an uncertain future. User growth has stalled, ad sales haven’t taken off, and adoption of its nine-month-old paid service, SoundCloud Go, has been weak.