I dont know if this is going to make sense, so bear with me, lol.
The way my first u/s was explained to me when it came to "hearing" the heartbeat was that at 7w what you were "hearing" was basically just a recording of the sound waves on an EKG, not the actual sound of the heart.
If that makes any sense at all, lol. That being said, I have no idea if this is true or not, but just they way they explained it to me.