Muffled sound from headset to another caller

[Umass021]

I had to get another droid because my speaker phone wasn't working correctly and now that I got a replacement people say I sound like I am under water or very muffled when I talk and they can barely hear me. I got have a seido case on it but it doesn't seem to cover the microphone. Is this a common problem with the droid? What should I do?