I had the same problem recently and I just got use to cranking up the sound. Than my main system died and had to start using my second puter. When I wanted to watch somthign on TV I would just plug into the headphone jack off my speakers pod. And wouldnt you know it but the sound was perfect. What I concluded the problem was that the output of my sound card was a line lvl signal. And really meant to be feeding powered devices. The inputes on my tv turns out are not self powered.
Try amping the signal before the TV, or going to a stereo first.
The headphone socket always gave me lower output on my TV.
Does your TV have a PHONO output on the back? - this delivers higher volume and a level output. Just get a cable with 2xphono -> 3.5mm Jack - the phonos go into the TV and the other end into the Line In port. Also just increase the Line In Volume in Volume Control.