I've experienced the exact same problem! I too had my Galaxy S4 replaced. The replacement is much better but still occasionally causes me to be muted to the person I'm speaking too. The problem has never occurred while on bluetooth or on speaker phone...only while holding the phone against my face. The screen turns off normally and it is definitely not a case of me accidentally pressing the mute button, although the effect is almost as if I pressed the mute button. Sometimes the other person can hear me very faintly. The consistent way that I "fix" it is to pull the phone away from my face for a second and then back and the other person immediately hears me normally. Check if this "fix" works for you too? There is definitely something wrong with the phone. For me it's not severe enough to want to deal with moving over to a new phone at this time.