iOS 9 makes it quicker than ever to start listening to audio from your iPhone.
As soon as you plug headphones into the headphone jack, the iPhone will take a guess at what you want to listen to and present the play controls for the suggested audio app. You can hit the play button right from the lock screen to get started, or swipe up from the app icon in the bottom left to launch straight into the suggested app for more control.
The app suggestions are based on frequency of use, but also the current time and location. For example, your iPhone will learn if you always use the Music app at the gym or listen to Podcasts on your morning commute, and make recommendations accordingly.