it seems that this issue is, sadly, common to a lot of Droid/Milestone owners.
If you check the Motorola owners' forum, under the Milestone section, there is a "mechanical" solution!
Unfortunately it is not a problem of software...pls go to motorola owners forums and search for the thread: "Motorola milestone horrible sound quality issues please advise"
I tried it and it worked.
In poor words it seems that a metal band on the back of the ear speaker (under a plastic cover) is vibrating and causes the problems of fading voices, metallic voices and tunnel effect...
Putting a piece of paper/rubber/or something similar between the metal and the body of the telephone stops the vibrations and the voice turn back to normal.
(but please follow the link, there are also self-explanatory pictures - more easy to do than to explain)
In my experience it works!
Well, for some time it worked but I had to open the plastic cover again after some days to fix again the paper that moved from position and, now, I will have to do again 'cause my friends are saying again that they don't hear me.
So, it is not an "official" solution, but if you read deeply in the forums, there is a huge number of customers complaining against Motorola as the company is not saying anything about this matter, as if it doesn't exist!!!