Both tech companies have declined to comment on the report. Apple, however, has already rejected Spotify's assertions. It argued that Spotify was really driven by "financial motivations" and wanted to benefit from the App Store's infrastructure without contributing to it. The firm also rejected notions that it was privileging Apple Music on a software level, going so far as to say it was talking to Spotify about AirPlay and Siri integrations.

Developers in the App Store not only have to provide a 30 percent cut of their subscription revenue in the first year (15 percent in the second), but can't steer customers to an outside website to subscribe there. That leaves providers either charging extra for App Store-based subscriptions (as Spotify does now) or eating a large portion of their costs.

There's no certainty that an investigation will lead to what Spotify wants. A victory for the company would have a significant effect on Apple, though. It might have to reduce or eliminate its cut of subscriptions, or ensure that its devices can work as closely with third-party media services as they do with Apple's own. While potentially liberating for users, it could eat into Apple's increasingly important services revenue and force it to rely on money from its burgeoning music, news and video offerings.