I first saw the heartbeat at 9w3d. Flickering on the screen. That was good enough for me! Hearing it doesn't make it more real than the flickering. So if you saw the flicker you are good :)
At my NT scan they flipped a switch on the u/s machine and we heard it, but i assumed it was the machine making the noise. Last week I heard it on the doppler and also heard some movement going on...that was cool too!