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var DirectSoundObj:IDirectSound8;
var FGenericBuffer:IDirectSoundBuffer;
var FBuffer:IDirectSoundBuffer8;

...

Res:=DirectSoundObj.CreateSoundBuffer(DSBufDesc,FGenericBuffer,NIL);
  if Failed(Res) then
    raise Exception.Create('Konnte DirectSound-Buffer nicht erstellen!');

Res:=FGenericBuffer.QueryInterface(IDirectSoundBuffer8,FBuffer);
  if Failed(Res) then
    raise Exception.Create('Konnte DirectSound-Schnittstelle nicht ermitteln!');