You can try these few option but no guarantee whether it will works but at least you got nuthing to lose:

1)Rapidly plug and unplug the headset into the jack.
2)While making a call using the headset, unplug the headset.
3)Plug in your heaset and check whether you have any song playing, stop the song and unplug the headset.
4)Make sure you have remove the earpiece plastic cover.

If all these fail, most probably hardware problem

Doesn't sounds good to me as I google and several reports that hard impact or accidentally drop the phone may have cause the problem. Hopefully Malaysia have someone expert who can fix or repair iPhone otherwise you need to send it back to the States for warranty.

I'm not trying to spread false info sorry, Actually I'm trying to mention to get it fix by people in US as they can easily purchase the iPhone parts. Only if our country have such expert who can fix our faulty iPhone...

This post has been edited by Joelles: Jul 10 2008, 11:27 AM