Ok, very well :)
Is there an easy way to check whether the FFT worked correctly?
Before applying the FFT I split the audio-material in chunks of 4096 bytes (to keep some kind of time-domain).
I created a sine-wave at 100 Hz and looked after applying the FFT in the double-array whether I could find the high magnitude but I couldn't. To find the correct entry in the array I calculated the frequency-band of every array-entry (44100/4096 = 10,7Hz) and looked in the corresponding part of the array where I suspected to find the high magnitude of 100 Hz.
I'm explaining this because I hope you see my error.