iPhone plays music through speakers even though headphones are plugged in

Hi!

So I've got a problem.. I took my iPhone 4 apart the other day to replace the home button. When I put it back together something went wrong because when I was going to listen to some music with my headphones, the phone played the music through the speakers but not through the headphones, even though they were plugged in. I have no idea what I did wrong, I've tried to google everything that I can possibly imagine but found no help.

Can anyone tell me exactly where and what the jack is connected to?

Any help is appreciated! thanks