No audio playback. CPU overload without reason!

I seem to have loads of problems on Cubase 7. Firstly some audio tracks just do not send audio out at all. You see the metering and see that there is some playback activity but there is no audio output. i have changed the track routing but that still does not resolve the issue.
i also have the cpu overloaded all the time even though there is only one audio file in the project and i work with a quad core 2.8 ghz mac pro. i have never experienced such strange problems in cubase 5. i have to say i am extremely unhappy with cubase 7.

Post specs and just a little more detail otherwise this will just look like another nuisance post. No good standing at the side of the road crying because your car won’t start. The mechanic needs more details than “Won’t go mr. Brm brm’s broken.”