YES! This is my problem exactly...
I called Motorola to ask if they had any kind of technology in the phone that would lower the volume based on ambient resonance, or whatever, and the representative said "no".
After that, he offered me a replacement, commenting that it was the first time he'd heard of such an issue, and telling me that it wasn't normal.
What do you all take from this? This issue is big enough to cause me to return my brand new, otherwise amazing, Moto X because I really need a reliable speaker.