First off, is this a fresh install of Vista or an upgrade. If it's a fresh
install then you need to install the audio Card drivers. Like Cal Bear said,
look at your device manager and see if there's red X or caution triangle with
an exclamation point in it. The red X means the device is disabled and the
yellw triangle either means there's a conflict or the proper drivers are
missing and need to be installed.
Your computer doesn't nessesaraly "detect" your Speakers, it detects your
audio Card. Its up to the audio Card to sent the proper signal to the
Speakers so there really isn't a set of drivers for Speakers.
I remember pulling the Hair out of my Head once when I couldn't get my audio
to work after moving the PC. I even re-installed the drivers. Come to find
out, I was plugging the Speakers into the old M-Board audio which was
disabled. The new audio Card was down at the Bottom and hard to see, I was
going by the colored Plugs and had a Forest Gump moment. Once I pluged the
Speakers in the right Card all was well, LOL!
Make sure you're plugging the Speakers into the right Plug, it could be as
simple as that.