Littlebits said:
Could also be related to my Realtek sound card.
Yep. If you're playing a stereo file through four speakers, you're going to have imaging issues. (not including the sub obviously, as it doesn't affect the imaging) I don't know how you have your sound card configured, but it may even be trying to derive multi-channel information from a two-channel source, (*Pro-Logic) or applying a pseudo-surround DSP algorithm.
* It's kind of like listening to a CD through a Pro-Logic decoder. It's going to be trying to send the mono signals through the center speaker, stereo information through the front left and right speakers, and ambiance (reverb and other effects) through the surrounds. Depending on the source music, this can either sound like a nice sort of pseudo-surround sound, or a pile of crap.