I'll try to explain this best I can. We recently got a Droid, and had the salesman install the snap on style plastic two piece case at the time of purchase. We were told by people we called that they were hearing a lot of their own volice echoing. We did some testing and found out anyone that we called with the Droid was hearing their own voice repeated to them loudly on their phone. Didn't matter if it was cell or land line. Also discovered that turning the earpiece volume up or down on the Droid would effect how loud the other persons voice would echo on their phone.
We tried everything we could think of, so we went back to Verizon, where they checked the phone, then replaced it for us. They swapped everything over for us, including the plastic phone case. We got home, and it did the same thing.
By luck, I took off the top half of the phone case and tried it. Echo gone! Put it back on, and the echo came back. Took it off, echo gone. I ended up dremeling out the little intent a little at the bottom of the case that sits atop of the mic. Works perfect now.
I still can't figure exactly what it was doing to cause the problem. All I know is, if you are having some weird sound quality issues, might be worth taking off the case and testing it.