How can I lower the media volume on my iPhone when no media is playing?

The control center works (swipe up from bottom of screen and slide volume slider as desired).

In iOS 9 at least, you can also go Settings -> Sounds and under the Ringer and Alerts section, turn off "Change with Buttons", then use the volume buttons on the side of the phone to adjust the media volume. You can leave this option off to always have media volume control with the hardware buttons if you are OK with the ringer volume being just off or on at a preset level using the silent toggle switch on the side of the phone.

If you are using IOS7 or 8 -- slide you finger up from the bottom of the screen to bring up the command centre. This should have the volume control on there so you can adjust the volume at any time.

to fix this go to settings then sounds and under the -ringer and alerts- section you want to turn the volume up and then unselect the option under it which is -change with buttons-

what this does is allows you to change volume with buttons without affecting ringer and alerts volume