first the obvious: if their powered check its plugged in and switched at source and the unit.
Then: volume at unit and speaker panel on computer (double click speaker icon in corner) check outputs are not muted.
Then: you have not pluged the sound jack into the microphone or line in hole.
Then: you have not pluged the jack that is sopposed to run in serial to another speaker, into the pc jacks.
Then: check within system / Device drivers / Sound... there are no conflicts.
Then : open windows help for further guidance.
Neil West London