Yes, you can fit one PCI card in there. But it will be about 1/4 inch from the top GPU fan, which may cause airflow problems.
If you ask because you want to use an existing sound card, I recommend you give it up and buy a PCIex1 sound card instead.
Maybe this: http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Creative SB X-Fi Xtreme Audio 70SB104000000 7.1 Channels PCI Express x1 Interface Sound Card