Hi again, I have a small update regarding this topic.
I have not 'yet' resorted to install the sound drivers from ASUS website, but I've been having no issues ever since the time I first opened this topic - I've uninstalled that day Adobe Flash Player (uncertain about which version, but it was for IE10), and so far, 10 days and ongoing, I haven't had any sound issues whatsoever. Reading through Adobe website also seems to indicate there was some sort of sound problem within the same timeframe, April - June.
Unfortunately for me, that wasn't the only thing I have uninstalled that day. I've also uninstalled MSI Afterburner and HWiNFO which were both running at Windows start-up. I especially think the conflict is between the audio/video recording feature of MSI Afterburner which was always turned on for video capturing whenever I wanted, and Adobe Flash Player for whenever I wanted to watch some youtube video - something to do with microphone or audio capturing, and I know both applications (Afterburner and Flash) do resort to audio capturing devices.
Maybe I'm talking out of my ass and nobody's here to even care, but those are just my obvervations. I will be more aware next time I re-install these two programs, one at a time, then if I don't get into the same sound issues again, I will install the other one a week later.
I no longer believe it to be a driver issue anymore.