My iPhone speakers don't play sound but work when calling

When I plug in earphones or a speaker station the music plays as normal, but the speakers themselves won't play. No music, no youtube, no alarm, it will vibrate instead.

However, if I call, I can perfectly hear the other person.

The phone isn't in quiet mode and the flip to the side is in the right position (no orange visible)

I have an iPhone 4

Does anyone have a solution? I tried restarting and resetting (homebutton+lockbutton)