You can use a HDTV USB adapter like....
http
/www.newegg.com/Product/Product.aspx?Item=N82E16815260023
However, it does not have separate cables for sound. I have never used this device.
What I would do in your situation is check your cable box (or whatever device you have connected to the HDTV) to see if you have coaxial out. If so then connect the cable box to both the HDTV as usual (HDMI cable), and a coaxial cable from the box to the above device.
Hopefully the cable box can output to two sources (check with your cable provider / Dish Network or whatever...). Alternatively your cable box may have audio output and your HDTV may have audio inputs. Simply connect the cable box to the device I suggested using coaxial cable and...