It is very difficult to he

It is very difficult to help you without seeing your setup. Have you checked those Audio output preferences yet? Perhaps audio is set to output to your computer speakers and not via Firewire.

Also, check the audio settings on your camera to make sure audio recording is enabled via the Firewire input.

Is there any way you can send another audio signal to your camera, either through another piece of software of from another camera or device? This could eliminate the camera or the computer from being the source of the problem.