I was entirely confused for a moment- I think you might be getting an echocardiogram, rather than an electrocardiogram. If you could hear an electrocardiogram, there would be something seriously wrong with their machine- It's meant to be a passive electrical measurement. Echo on the other hand is exactly what you described, an ultrasound of the heart.

I was actually thinking you might have a strong interoception, which is when people have an awareness of their own heartbeat signals- super rare but super cool.