Speaking at a technology conference organized by The Wall Street Journal in Laguna Beach, Calif., Cook said that an additional 8.5 million people are participating in a free trial of the Apple Music service. “I’m really happy about it, and I think the runway here is really good,” Cook said. Released in June, Apple Music is the company’s attempt to carry its dominance of digital music through its iTunes store into the era of music streaming pioneered by Spotify and others.