Check your audio settings in your stb. Airtel sends the audio as is from the broadcasters. It's a major disadvantage for normal TV viewers but it's an advantage for home theater users.
It may be because of audio channels. If we watch sd channels directly we will get stereo out put, for hd channels is having 5.1 audio. We can easily notice those HD doesn't have 5.1 audio. Example ETV HD 880.