In my Inspiron 14z, when I plugin a set of Apple earphones, it is identified as a microphone, because Windows shows a popup in notification area saying that "Windows can now play sounds FROM this device."
That being said, I can't see the headphones icon in volume options, so that I can adjust its individual volume settings, and reduce the earphones volume when I plug it in.
I already try this, and it didn't work: http://en.community.dell.com/support-forums/laptop/f/3517/p/19497223/20324248.aspx#20324248 . And all other solutions that I find seem to go along this line.