In a room this quiet I assume using keyboard will sound very loud.

Not to mention the ventilator, the HDD, and the rest of the electronic components.

Quite the opposite - hence, 'quiet room'.

How do you hear your heartbeat if you can cannot hear your keyboard keys when pressed?

Because the veins are very near your ear canal. Any outside sound is not available, so the only sound you hear is those that are internal and wouldn't get very far anywhere else...

And I imagine even them are not louder, just that you're a lot more focused on them.