I have 2006 GS300 with ML audio system.Today i drove car,suddenly volume from rear speakers went almost down,its way more quieter than front speakers,is it
rear amplifier or what is wrong.Somehow i think rear voice is also worse quality when front voice..?
Sorry my bad english.....

 

Take the grille off and look at the speaker. The ML speakers are not terribly robust, and foam fails and they separate. It may be just that. When voice becomes scratchy, that’s a strong sign of that. They can often be repaired (glued), or replaced cheaply - no need for factory speaker really.

 

@Kawasage , I have repaired both the subwoofer and the rear surround speakers and while I agree with @Oro you could start by just looking at the speakers and see if there's any physical damage, DO NOT attempt to just remove the grill.
It'll not just pop off, you actually need to remove the rear seats completely to remove the grill. You'll find some tutorials on YT. It's not hard to do it yourself, but it will take you about an hour or so.

If the speakers foam failed they can be easily replaced at a very low cost.

 

I have listening couple days and it seems like that music voice from behind is really normal bass,medium,treble but strength is way less than front speakers,ratio is about
1:4,i dont think it is speaker,maybe some amplifier or something else is bad.
Still i have to check rear subwoofer,maybe....

 

Maybe it sounds silly but have you checked the sound balance? Maybe it's all front.

 

I hope it would be that but no,if you turn to back you hardly hear music..