Ah, I understand.
FMOD uses by default FSOUND_SPEAKERMODE_STEREO as speaker mode. Try using
fsoundSetSpeakerMode(FSOUND_SPEAKERMODE_SURROUND);
and report what happened. Just add the line into the 3D demo somewhere in the main function. I can't test it because I have no 5.1 soundcard...
I noticed by your checking the function that I missed to add the speakermodes to the manual, thanks!
Best regards,
-Christian