Output From Computer Speakers Plugged Into Tv Super Low?

I just bought speakers for a computer that include 2 speakes and a sub. Well my highspeed lcd tv has a headphone jack on it so when I plug it in the output is super low and there almost no output comming from the sub. I plugged the speakers into my laptop and they sound great. Is there less output coming from my tv or is there something I can do. Its a 32″ Olevia lcd with 20 watts power for the speakers. Also it has a/v outputs for it so if I buy a y-plug will it make a difference then?