If your Play Mixer (Volume Control, sndvol32.exe in Windows) isn't shoved hard over to left on the Balance pot on the Line In section, you could have a bad connector. Try using the AUX connector on the card by wiring a 3.5mm stereo jack to an old CDROM audio adaptor cable and see if it records right with that; if so, either just mount that jack as your new Line In on the case (back or front, your choice) or replace the jack on your sound card.