AC3 Encoding problem

[Sherlock Holmes]

Hi guys
I have a huge problem with AC3 recording.
I am a movie dubber. I record the dubbed voices in WAV format , then mix them , and at last I encode voices and other sounds of movie to AC3.

Last week I dubbed "The Big Red One"
I don't know why when my DVD (or AC3) is played with a decoder , the sound is corrupted. In this case , everything is ok , but when the dubbed voices starts , it goes very low , and when it finished , the sound goes back to normal.
This problem happens when my DVD or AC3 is decoding with decoder. But when I hear my AC3 or play my DVD in downmix mode , everything is ok.
And also the WAVs of voices are completely clear and ok and plays very nice like a bitch.

I am confused , please help me before I screw up.

Thanks in advance