In the 1950s I was training in both Judo and Aikido. In Judo we had to slap the mat as your opponent never lets you go, often throwing you into the mat...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 ?
Henry Ellis
Aikido Books
http://aikido-books.blogspot.com/