Here's a potential no-cost solution.
If you loaded the drivers for your sound card from a CD that came with your motherboard, or worse, you are using drivers supplied by your OS (Operating System - Win2K, WinXP, etc. - these drivers would likely be REALLY old), then it is possible that newer drivers just might give you adequate performance (no guarantees).
Therefore, if you haven't already done this, I suggest you:
(1) Identify the sound chip used by your motherboard. It should be identified in the documentation for your motherboard, or the driver documentation for your motherboard, or at the motherboard manufacturer's website, or by the Device Manager for your OS. If all else fails, then just look for the sound chip on your motherboard (its location should be indicated by a schematic drawing or labeled photo in your motherboard documentation) and get the model designation directly from the chip (youl may need a magnifying glass of some kind to read the model designation on the chip).
and
(2) Go to the sound chip manufacturer's website and see if there aren't more recent drivers for the chip. If you install those drivers (if you do this, of course remove your current sound driver first) and if the sound quits working or degrades after these (newer) drivers are installed, then remove those drivers and go to the motherbard maker's site and see if there aren't more recent drivers (than you are using right now) at that location. If there are, then remove whatever sound drivers you are using at that point, install the newer drivers, and test.
Another possibilty (in fact, you should probably try this first if you haven't already done so):
If there is a sound configuration utility for your sound chip, then make sure your sound is set for "headphones", and try tweaking the "headphones" sound using that utility. Maybe you'll get lucky.
I hope none of my suggestions insult your intelligence but I have no way of knowing how knowledgeable you are about PCs.
Good luck.
Edzard