Simple fix is to go to Radio Shack and get a couple of 1/8" to 1/4" Stereo adapter plugs, and a 2 or 3 foot long 1/8" stereo extention cable and plug one end into your Soundcard's Line/Spkr output, and the other end to a Line-INput on yer stereo system, Tape deck, etc.
Then, plug yer headset's plug into the female end of the 1/8" female to 1/4" male stereo HP adapter. Plug that into the amp of your choice's HP jack, and get ready to lose yer mind if ya ferget to turn it DOWN 1st! LOL!
Whole setup w/ a couple a spares for other adapter projects, will run ya less than $10 American I s'pose.
Even if you dont own a world rocker Stereo main amp, Running from PC out to REC/Line in on a standard cassette recorder will give ya up to 20 watts boost w/a volume control.
Good luck;
Have fun, or else!