You may have previously tried it, but give it another shot and see what happens this time. If your iPhone 6s is stuck in headphone mode due to a software problem or glitch, a soft reset would most likely fix it. On the iPhone 6s, here's how to make a proper soft reset:

• Until the slider displays, press and hold the Power (Sleep/Wake) button.
• To turn off your iPhone fully, drag the slider.
• Allow roughly 30 seconds to pass.
• Press and hold the Power button until the Apple logo shows after the timer has expired. After that, your iPhone will reset.

• To force a restart, press and hold the Power (Sleep/Wake) and Home buttons at the same time until the Apple logo displays, then release both keys. Wait until your iPhone has completely rebooted before checking to see whether it has already returned to regular mode.