It would help if you would add the year/model to your description as equipment varied over the years, as well as more info on how you cabled up the new TV.
In general, many configs used a fixed-volume surround sound system that depended on the TV to regulate the sound volume. There was no volume control for the surround sound amp.
There are a couple way to fix this, depending on your system. I'm assuming you have plugged into the "audio out" on your TV? If so, see if your TV allows (in the setup screens) to change the audio out from "fixed" to "variable". Some do, some don't. If you don't have that, you can try plugging into the headphone jack instead of the "audio out" if you have one on your TV. This will usually disable the TV speakers but let you control the volume to the surround sound.
If both fail, you are looking at a more complex solution of directly tapping into the TV speakers. There are several threads on how to do this.
Hope this helps, am just guessing at your config.