Great great news! I'm so happy that I've fixed that stupid com speaker problem!
No guys, it wasn't all the things you suggested... I had done the unplugging and plugging stuff many times last time until I had got tired of it. Checked all my audio settings... anything with volume was set to maximum, nothing was muted except mic (deliberately). Everything was ok.
Then I went to see the cable and it was unplugged (no, this wasn't why it didn't work. I unplugged it last time after doing the unplugging and plugging thing and got so sick that I just left it unplugged cos I gave up). So this time, I pulled out the cpu tower cos I wanted to check which port to plug it back in (cannot remember already) and I plugged in the cable... and it worked! Sound came out from hi-fi speakers!!!
I was delighted. Then I moved the cpu tower back into position... and I usually push it quite far back so that the cables at the back press against the wall cos I don't want the cpu to potrude out too much. Then, the sound stopped. So I did some adjustment with the cpr position and the cable wire... and the sound came back!
So by chance, I confirmed what I had suspected all along... it's the stupid cable messing up! Damn! I had suspected this all along cos I checked all my settings and also replugged them and just doesn't make sense why it can't work. The most likely reason was that the cable is faulty. But couldn't quite believe it too cos cables seldom get spoiled and I have used all my cables for years with no problem. Don't know what's with this cable... pissed with this cable now. It's quite a thick, good quality one somemore... not those cheapo type. Grrr... Maybe cannot bend too much near the head. But all the other cables on my cpu also work fine... Dunno why this one cannot bend... stupid cable...
Anyway, thanks to DM and Sammy for their troubleshooting advice. Although it didn't really shoot the problem, it got me messing around with it again and ending up solving the problem. Thanks!