The soundcard thinks the headphones are actually a sound system, so it is outputting sound in a "line out" fashion, not headphones.
See if there is a dedicated plugin for headphones.
If there is no connector:
Check the Realtek driver software and tell it to configure the plug for "headphones", it should be able to do this automatically, but realtek is cheap, and it prolly doesn't....
I don't know exactly how to configure your realtek. It could be any one of millions of different combinations.
Go into the windows control panel > sound and check out the properties and start poking around in there. You are looking for something that is saying "line out" that can be changed to "headphones".
I don't have a solution to this, but I came here after getting the same problem. I got a new Samsung Series laptop with Realtek HD audio, and the speakers are AWESOME, but using the headphones always gives low volume. This was not the case on my old Dell Inspiron laptop. So it's not a problem with the headphones. I think the solution would lie in somehow getting a USB headphones which are independent from the soundcard or something.