I just installed a soundblaster audigy platinum sb0090 card on win vista 64. I got the latest drivers from the site and the sound seems to be working fine, but.. there is only sound going to the headphones! I then go into the sound properties and under a custom tab there is a "digital output only" option which I tick. now sound is restored to speakers but nothing on headphones!! If I untick the headphone sound works and visa versa.
Don't know what to tell you since i've used soundcards and speakers on pc's since the 90's and not one set-up has required the use of digital outputs for speaker playback.
Maybe a more detailed description of your mobo and speakers might help.
Also i assume you've disabled onboard audio in your bios.