Big Difference Between Sound When Watch Tv (through Hdmi) And Watching A Dvd

I have a Polaroid TDAC-02212 that's connected to a U-verse hd box with an hdmi cable. The sound is so low when watching tv (either regular or hd channels) that you can barely hear it, even with the volume turned all the way up.   I know the volume works fine because if I change the input to dvd and try to watch a movie it's so loud I have to turn it all the way down to one or two bars.     I hooked up a set of component cables hoping that was the problem, but it didn't make any difference.   I'm hoping someone can help with this.  I am unable to watch tv at all with this set unless I'm watching a dvd.    thanks!