This passage comes from the Akallabeth:
But the fear of death grew ever darker upon them, and they delayed it by all means that they could; and they began to build great houses for the dead, and their wise men laboured unceasingly to discover if they might the secret of recalling life, or at the least of the prolonging of Men's days. Yet they achieved only the art of preserving incorrupt the dead flesh of Men, and they filled all the land with silent tombs in which the thought of death was enshrined in the darkness.
That last part could be taken to mean the Numenoreans mummified their dead. However, they could have just discovered a method of embalming similar to modern methods. Which option do you think is more likely?