There is a software glitch affecting the Lighting EarPods that ship with the iPhone 7 and iPhone 7 Plus. Some users have discovered that the volume control dongle stops working after leaving their EarPods plugged into the Lightning port of their iPhone 7 or iPhone 7 Plus. The bug stops users from adjusting the volume of their music, and does not allow them to access Siri or answer calls while wearing their EarPods.
Apple has acknowledged the problem and has said a fix is coming in a future software update. You can temporarily fix the problem by unplugging and plugging your EarPods back into you device.