Why are HD channels quieter than normal channels?

Question

For example, TNT HD is quieter than TNT itself

Answer 1

It's the way the sound is broadcast from either the cable or satellite company. I have directv and sometimes it goes the other way where the HD channel is louder.

Answer 2

HD channels use 5.1 channel Dolby Digital sound. It may be that the distribution of sounds into the five channels is deifferent than the normal TV channels.