Makes and model numbers would help.
You would need to have ARC in both the soundbar and TV, I suspect you don't in the TV since it's probably too old. If it does then you have to connect the soundbar to the HDMI-ARC input. There will only be one. Then you still have to turn ARC on in the TV menu and select it as your input on the soundbar.
An alternative is to connect the TV audio output to the soundbar. You might need a converter depending on the TV output type and soundbar input type.
Another alternative is to connect the soundbar directly to your source or to an HDMI input selector with audio extraction.