First off, What OS are you running?
If Windows 7, Can you open up the playback devices and look for how many devices are present and how many are enabled or 'ready'? Make sure that one of them says "Default". While I've never seen it where there is no default perhaps, perhaps, perhaps.
If this is Windows XP, I have forgotten too much of it.
But the above is Windows 7 steps are similar.
I wouldn't jump on the sound card at this point. How many games and which ones? Are these very old games, commercial games such as Diablo 2 or 3, or lower-quality games for the kids?