It's kinda a quirky setup. One thing I've noticed is, if you mute the laptop while the SB card is NOT in, you will hear nothing even after rebooting and installing the SB card.
The way I think it works is the SB card does all the hard work, then passes a sound file on to the onboard card for output. So, in a sense, BOTH cards are working when the soundblaster card is installed. If you've muted on-board, then installed the SB card, it becomes the default sound device, but the onboard is still muted and you won't hear anything. Make sense?
Here is a thread just about this pass-thru feature:http://notebookforums.com/showthread.php?t=69734