We have a new cable box, Scientific Atlanta

We have a new cable box, Scientific Atlanta Explorer 4250 HDC rented thru Comcast. the Cablebox signal is connected to a Bose Lifestyle® V20 home theater system, via an HDMI to HDMI cable. The Bose Lifestyle® V20 home theater system is connected to the TV (Sony Bravia XBR4) also with an HDMI to HDMI cable. We used to be able to conrol the sound with the remote for the cable, you'd see the bars moving and the sound would go up and down, and the mute worked as well. Since switching to the new Scientific Atlanta Explorer 4250 HDC cablebox, we can no longer control the sound with the remote for the cablebox. We still see the bars (from 0 to 30) moving, but the volume level does not go up or down. This is very annoying, I want to get back to the way the sound worked with the old cable box.