Your Vizio TV worked fine with the DTV box but when they installed the cable box you don't get sound on the TV anymore?
Did it work when the set you up for cable?
You can try rebooting the cable box.Unplug it from the wall for 3 minutes.
Also make sure the cable box menu is set to output audio via the HDMI connection to the TV.
Call the cable provider and let them earn their money.