Early Aikido was always silent throws as if you were being thrown on concrete as Marc has said...I am surprised to see that many Aikido dojos do a lot of mat slapping, maybe they like the sound ?
You'll find a lot of slapping ukemi in the pre-war footage of Ueshiba. Not saying it's better or worse (because I don't know)...only that slapping was certainly a part of early, traditional aikido. The Yoshinkan still follows this method. I'm guessing that not slapping was likely a post-war convention.