As John said, the DHS surround is a fixed amp system. It requires the source (TV etc) to change the volume. There is no remote or other control for it.
If you have the surround switched on, as you turn the volume dial on the radio, the sound level should change. Same with the TV... using the remote on the TV to change the volume should change the sound through the DTS system.
If the radio changes fine but the TV does not, then it's likely your TV was switched out at some point. A common mistake in replacing the TV is to just hook up the surround to the TV's audio out connections. Most flatscreens today have a "fixed" audio out because they assume you are connecting to a surround system that has its own volume control. This is not the case in the DTS system. There are various ways to get around this... checking the TV to see if you can change the audio out from "fixed" to "variable" it the TV's setup menu, using the TV's headphone jack instead of the audio out (requires an adaptor cable) or actually soldering the DTS wires directly to the TV speakers (requires opening up the back of the TV.) There are many threads on this.
So if your radio volume works to change the DTS but your TV doesn't, the problem is with the TV connection. If neither works, then something else is going on.