sound

Hi everyone, I just got my new Cubase 7 and am excited, I just can’t seem to get the sound working properly, the speakers are fine as I have sound when I play the keyboard, but when I play the youtube videos there’s no sound, there’s input activity on the transport bar, I have a Saffire 6 USB audio interface, can anyone please help, many thanks.

http://www.steinberg.net/forum/viewtopic.php?p=220615#p220615