How I test such is simple. I boot a LiveCD of Linux and play something...
i'm running XP Pro SP3 Intel PC
running rca leads from PC [creative usb digital music LX] to stereo amp receiver [2 speakers only].
tried output source from headphones also and same problem.
playback balance slider is always to the right side. i move it to the centre and it's okay for a short time both channels and then the sound on one speaker is almost totally gone again - very quiet - hardly audible. slider in the play control panel is still centred until i close it and re-open it. Always reverts to the side again.
this likely to be a driver problem?