If cross talk is indeed the problem, a new sound card would probably fix it. However, I was just speculating/guesstimating so I don't feel comfortable recommending you to do anything based on that. If you wanted to try it though, feel free. A basic card isn't nearly as expensive as you'd think, so you'd probably still be saving money. http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=2010360057%204093&name=%2410%20-%20%2425
Again, I'm just pulling at straws and can't be blamed if you buy a new sound card but that doesn't fix anything
Hopefully someone else can chime in with something more experience based.