I installed a new Contour box yesterday but there is now no sound coming from the TV through our cable... We have an Apple TV hooked up and there is sound coming through when we use that. All connect...
I'll assume a basic setup: Apple and Cox connected via HDMI to an HDTV.
Did the cable box initially have audio? You watched Apple TV, switched back to the cable box and then no sound?
It may be an audio setting in your TV. Apple TV may have set an unsupported audio format. You may have your manufacturer's version of HDMI-CEC enabled but since cable boxes don't support it, there may be an audio input glitch. A Samsung model(s) had audio glitches with multiple HDMI inputs a few years ago, but I believe a firmware update fixed it.
I doubt it's a hardware problem but instead a software setting. Check the audio setting on your TV, check for updates, disable HDMI-CEC, etc.