Cannot hear audio on iPhone 4s only when headphones are in

Ok, this is going to sound bizarre but here it goes. I own an iPhone 4s and I've suddenly developed a problem with the audio. When I play music, everything it fine however when I put in headphones, I can't hear the music at all (tried iTunes and YouTube) however I CAN hear the keypad sounds. I've tried a couple of different pairs of headphones and have the same issue. Any ideas?

Thanks!