Does the volume control of the cable box affect the volume of the TV (without having programmed the box remote to operate the TV volume).
I suspect that the HDMI output of the cable box is fixed volume. Check the box menu to see if there is a setting for this.
The work around is to connect the soundbar to the TV HDMI-ARC input/output. Turn ARC on in the TV menu and program the cable box remote to adjust the volume of the TV. This way you are actually using the TV volume to control the soundbar volume but are substituting the cable box remote to send those commands to the TV instead of the TV remote.
Another option is to get a Logitech Harmony universal remote that will control everything.