Addicted Newbie

I am new to flight sims and am absolutely addicted. I have FS X and the expansion pack. I did not buy the deluxe version and am regretting it now. I would like to know if there is a way to download more missions. I have succeeded in getting 2 aircraft but 1 does not show up in the menu, and the other has no sound and the throttle does not work properly(rpms go up but not airspeed). Any help would be awesome!