This laptop has only one jack for speaker/earphone and mic. So basically, its input/output jack. I've tried to use a mic here and it works fine, so the only problem is the output.
Luckily, after a long journey in the web, I found a solution to it.
Just simply download alsa-intel-hda-optiplex-
That's it! Asus K42J and Ubuntu 10.04 now works in harmony :)