"They're used to being a shaper rather than a responder," Mr. Sacconaghi said. "This is one of the few times where Apple is playing catch-up and not necessarily coming from a position of strength."
Tom Neumayr, an Apple spokesman, declined to comment or make executives including Mr. Reznor available for interviews.
Apple's turn toward streaming is a matter of necessity, as listeners increasingly shift from music downloads to streaming. According to the Recording Industry Association of America, downloads generated $2.6 billion in revenue in 2014, down 8.5 percent from the year before. Streaming made $1.87 billion last year, and overtook CD sales for the first time.
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As the biggest retailer of music, Apple remains a crucial marketing partner for the music industry. Yet its absence from streaming has let others get a head start. Spotify, which started in Sweden in 2008 and came to the United States in 2011, said in January that it has 15 million paying subscribers around the world, as well as 45 million more who listen free, with advertising. (Apple's iTunes has more than 800 million customer accounts.)
Exactly how Apple will match Spotify is unclear. Music executives say they have not been shown a prototype of the new streaming service, nor been given much detail on it.
The new music app, which is a collaborative effort between Mr. Reznor and other Apple and Beats employees, including Jimmy Iovine — who founded Beats with the hip-hop star Dr. Dre — will feature the streaming music service with many of the same characteristics as the Beats Music streaming service, one Apple employee said. Those may include curated playlists and a more vivid visual appeal, while conforming to Apple's sleek and minimal design aesthetic, one person said. The name Beats Music will most likely be shed.
According to an Apple employee, the service is being tested as part of a new version of the company's mobile software system, iOS, which has been given the code name "Copper" and is expected for public release this year.
Mr. Iovine has set the tone of the transformation of Apple's music plans, according to music executives. Mr. Iovine, who reports to Eddy Cue, Apple's head of software and Internet services, has been leading aggressive talks to secure prominent album releases that will be exclusive to Apple, akin to what Beyoncé did when she released her self-titled album on iTunes in December 2013. One music executive involved in the negotiations described this part of the new iTunes as "Spotify with Jimmy juice."
A crucial difference for Apple's streaming service is that unlike Spotify, it will have no free tier. That has greatly pleased top executives at major music labels, who have begun to complain openly that so much free music has given consumers too little reason to pay for it.
Apple is also expected to overhaul iTunes Radio, the free service that the company introduced in September 2013 as a competitor to Pandora, and which has had little impact on the marketplace. One new player is Zane Lowe, a former BBC radio D.J. known as a trend-spotter. Last month he announced that he would join Apple in Los Angeles, where the Beats team is concentrated.