iphone 4 audio jack not functioning properly

i plugged external speakers into my iphone 4 audio jack the other day and then it stopped working. the phone speakers are still playing sound and the volume control buttons are working fine too. i then replaced the audio jack with a new one and i still have the same issue. there is no sound output over the headphones, but the volume control from the headphones and the headphone mic are working alright. what could be the problem??