Logic Express 8 and Kontakt (NI)

Hi, everybody,

I am trying to use Native Instrument’s Kontakt sampler via Cubase Studio 4 (Steinberg).
Some instruments work fine (e.g. rhythm guitar) but most do not, i. e. I can see that my keyboard triggers the notes, but I cannot hear them. I use M-Audio Fast Track Pro audio interface, everything works (the rhythm guitar being the proof)+ why can’t I hear all instruments??????
A solution much appreciated...
Peter