You're already getting audio from the TV over HDMI. Your headphones aren't designed to play actual 5.1/7.1 positional audio based on the fact that they use processing to replicate it. That's the whole "virtual surround" part of it. In order to play full 5.1/7.1 audio as it was originally recorded or designed to be reproduced, you'd need a set of headphones built for it. There are a few different manufacturers out there who make them. I can't guarantee that a dedicated sound card is going to help you.