Sounds like you've hit all of the usual diagnostic bases.

New speakers cost about $20 and sound pretty good these days. You could pick up a new set and if they don't work then the speakers probably weren't the problem and you can take them back.

Changing out the audio card is also pretty easy and they probably don't cost a great deal these days. I'd check to be sure everything is plugged in tight inside the computer before buying the extra speakers or buying a new card, just to see if that makes a difference.

Opening your computer is easy and there is a lot of space in most of them for checking and/or replacing the card. Just don't touch the contacts on any of the stuff you're handing and avoid doing it at a time when there is a lot of static electricity in the air. (And the usual precautions apply--unplug it first.)

SRS