Music plays through phone speaker instead of headphones when app is opened?

This problem started a few days ago and it's extremely annoying in quiet environments. I will be listening to music from any music player app, and whenever I open an app, get a notification, or switch the song the music starts playing from the phone speaker instead of the headphones. How can I stop this? Is this a software issue or a problem with the headphones itself? Is it just something in the settings? I've tried a lot of different methods. Thanks!