Setting the threshold on stimtracker audio input

I am using stimtracker to detect the onset of an auditory stimulus and marking the time of the onset in an EEG record using Biosemi. The auditory stimulus is generated in matlab on a PC and output to Stimtracker via the built in sound card. Stimtracker is only able to detect the audio input when the threshold is set to 0. Any increase in threshold precludes detection despite the fact that the volume on the output is near maximum.

Can you post one or two sample audio files?