How can I fix muffled sound on my Samsung Galaxy S? I recently purchased a i9000 and the voice quality over the microphone is horrible. The sound is muffled. Person on the other side of the line cannot make out what I'm saying. Is there a fix?
Pop off the back cover and make sure there's not a sticker or anything over the speaker or mic.
You could try Voodoo Sound.  It's a custom kernel for SGS devices but if you don't want to mess around with that you could just try the Voodoo Control app, it can do some improvements without the kernel.  (If you do use the kernel you really need the app as well.)
Otherwise, I would assume it's a hardware problem and you should get a replacement.