Sound output from win 10 device

I need my computer to generate a beep sound thru headphone jack to trigger an audio relay switch to commence a race timing countdown clock. The issue is that when the race software is maximised on my screen the audio relay switch fails to pickup the audio signal from the jack. I have checked that there is still sound playing in the headphone jack even when race software is maximised but when the software is maximised the audio relay doesn't work. I would appreciate any help in cracking this issue because we need the race software maximised at the same time as the beep. I even tried getting the race software to generate the beep but the same thing happened Thanks John