Erik Malund,
Sorry, it was a typo. I connected it the way you mentioned. Thanks for correcting me.

For now i think i have found the root of the problem; the Base current. Previously as Leonardo mentioned, he suggested that a 10k resistor is connected to the base of the transistor but all i get is a very soft hissing sound from the buzzer. However,using PNP transistor, i reduced the resistor down to merely 27ohm and after connecting the Base to GND (real ground), to my surprise it works out perfectly! The sound is so loud and clear. I tried using 100ohm, and the sounds start to distort again.

However, different results occur when i connect it to the 'virtual-ground' (im not sure if that's the right word); the low logic (0V) of the P1.0. I think the reason behind this is that P1.0 could not source enough current to drive the transistor such that a full current flows through from emitter to collector which is very much needed by the buzzer.

Please correct me if i am wrong and suggestion to overcome this problem.