you might need a soundcard which supports dolby digital live. i know this one supports it http
![Disenchanted :/ :/](/styles/bom/smilies/disenchanted.gif)
/www.asus.com/us/Sound_Cards_and_DigitaltoAnalog_Converters/Xonar_DX/specifications/ but if you have a look around you can likely find a much cheaper option (it should say supports DDL or dolby digital live). i think the u3 supports it (but i think it uses a coax output) as well but not sure about the audio quality on that one.
howver before you make that plunge... research if your home theater supports more than 2.0 via optical or coax. some HTIB sets only take 2.0 from inputs not surround. before buying a soundcard make sure you read about some people getting full surround sound from optical or coax inputs on your exact set to verify its not a problem that wont be fixed by getting a soundcard.
(as it could be either or causing the problem)