If you have one of those headpones that don't indiscriminately leak everything between the channels like mad (called crossfeed - some is desirable, but we want to control it ourselves), this guide is for you. If you do have one of those headphones, this guide is not for you. Refer to the end of this post for a way to test for it under acceptable conditions.
Anyway, here the guide:
1: Download the Bauer Stereophonic to Binaural-plugin
Get the version for your media player here if it's listed. If your player is not listed, see if it supports VST or if there is a plugin available for it that supports VST. Mediamonkey for example can use Winamp's VST-wrapper. In that case you download the VST-version.
Linux-users who do not use XMMS likely need the LADSPA-version.
2: Install your plug-in
It's as simple as it sounds. No explanation necessary, I think. I don't know how it works with Linux, but Linux-users probably do know their way around.
3: Activate and enjoy!
Now you've done it! If everything works, you should experience that unnatural one-sided sounds have absolutely vanished, and depending on the settings you choose you may notice an overall different sound, which you may or may not find enjoyable.
For a very basic grasp of what the settings do, refer to this (slightly improved) paragraph from the BS2B homepage:
1) 700 Hz, 4.5 dB - default.
This setting sounds closest to virtual speaker placement with azimuth 30 degrees and a distance of about 3 meters.
2) 700 Hz, 6 dB - most popular.
This setting is close to the parameters of a Chu Moy crossfeed.
3) 650 Hz, 9.5 dB - making the smallest changes in the original signal, only to aid relaxed listening with headphones.
This setting is close to the parameters of a crossfeeder implemented in Jan Meier's CORDA amplifiers.
Of course there are buttloads of other ways to achieve this, but I am not acquainted with them, so yeah.
*If you want to test if you have one of those bad leakers, what should work well is using a test signal dedicated to one channel and listening whether it actually just comes from there.
Follow this link and search for "wiring", try the left and right channel test signals there.
If there is no serious crossfeed, you'll notice, trust me. If your brain doesn't get irritated by a completely one-sided sound, then there's definitely an impractical amount of crossfeed which should render these tips less useful if not completely useless. Take my words with a grain of salt though as I am not accustomed to your situation and not a professional working in the field either, maybe there is an appreciable effect anyway. Trying never hurts.