You can get a tuner card but the only HD channels you'll get are teh unencrypted ones andmaybe the over the air ones if your cable company puts them in the wire like mine does.
To get the other HD channels you'll need to purchase a card that accepts a cable card (these were like $200 when I checked) and then rent a cable card from your cable company (prices vary).http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007683%20600160489&IsNodeId=1
Looks like they're still almost $200
As for the sound, you dont have to pump it out the hdmi port. You can select your soundcard as the primary device as long as its hdcp compliant.