Look in Device  Manager > sound,video and  game  controllers for  any
conflicts, if  there  are it's  likely  you  will have  to  reinstall the
drivers  for  your  sound card. In that  case, restart in safe  mode first
and  remove  all references  to  the  sound  card in DM, reboot  and
reinstall the  drivers.
If all is  OK in DM,  goto Control Panel > Multimedia, open the  audio tab
and  check what  the  preferred device is  - it  should  equal your  sound
card, whatever brand it  is.
One other  thing, and  I am not  being  facetious here, but  is  the  cable
from the  speakers  inserted into  the  correct  socket on the  sound
card?.
Newer PCs have colour coded connectors - lime green is for the  speaker
cable, on older  sound  cards they were all [usually] black. So if  you
don't  see "spk out " on the  back plane  you  may have to  try all the
sockets to verify which is the  correct one.

rgds
Li'l Roberto