How to choose music not from the iTunes store on iPhone to use for apps?

I have several video/photo editing apps on my iPhone 5s, most of which allow me to import music to use as background music. When I click 'add music' on these apps, it says, "There's no Music in device". I have music, but it's just not from iTunes (since I'm cheap and don't want to pay two dollars for one song). Is there any way I can change the settings on my iPhone so that another app is my default music app? Or is there a way I can move songs into the preinstalled "Music" app on iPhones that only allows you to have songs from iTunes in it?

Actually you can add any mp3 song you already have into your itunes list and copy it to your phone.