Is the source your computer or what device is providing the sound to the system, and are you playing music, games, movies, what??
If it is your computer, have you set it to 5.1 mode?
Most theater systems have a spot where you can set the individual speaker to + or - dBm. Depending on the source and content, it might just be that the background noise is minimal and so you dont hear much. In a 5.1 setup the center is for voices, left/right for foreground sounds, and the rears are for background, and obvioulsy the sub is for low frequency.