I've gone through all kinds of irritation and expense over this issue, and maybe I can save you from banging your head against a wall. The combo port will NOT work with an external, standalone mic. You CANNOT fix this with any kind of splitter. You CANNOT fix this by endless fussing with the audio settings, in either Windows or Realtek. Asus and other laptop manufacturers have BROKEN the old mic port, and they did it on purpose because they imagine nobody wants to plug a real mic into a 3.5mm jack. They are wrong of course, but there's no going back. Just don't let them lie to you and waste your time.
What does work? Get a USB mic, a decent one. The Blue "Snowball" is so good I have to turn the gain way down. Or get a full-featured external USB sound-card (not the Turtle Beach Amigo II, which unfortunately does not work for this). I got the Creative Soundblaster X-Fi Surround 5.1, which works with any regular mic I plug into it, with zero noise -- plus it can do line in, RCA plugs out, 5.1 surround and optical digital out.
I hate what Asus (and others) did with the stupid "combo jack" but they'll never admit how horrible it is, so don't make yourself nuts -- bite the bullet and go USB. Or, maybe give up having any "real" mic and try a headset that's made for the ridiculous combo port. Or stoop to using the built-in mic, if your laptop has one -- it might not be as bad as you'd think..