1. Test it in other ports on the TV. If the sound works in the other ports, then the issue is with the port.
2. Test the DSTV decoder with other HDMI cables. If you get sound, the cable is the problem.
3. Test the original cable with other devices that use HDMI (laptop, etc.). If you get sound, the decoder is the problem.
4. If all else fails, redo the tests with another TV set. If you get sound, it's the original TV.