I was using Airtel on HDMI cable and there was no such issue. Because, Airtel doesn't do anything with the Audio. They transmit as is from broadcaster. However, TS and Sun Direct are fiddling with the audio in the middle by trying to upscale it to Dolby Digital Plus which is why there seems to be an issue. If I'm right, Optical does not support advanced sound technologies and hence gives the original Dolby Digital audio which is why the volume and quality would be more on it. Let me confirm it tomorrow once I receive the cable.