PC connection to subwoofer problem

Hi there,
I have a Dell OptiPlex which is not giving audio output when connected to a subwoofer, sound drivers are okay, audio test on the control panel is working okay but when I try to play media files there is no sound at all, I have checked the application settings and volume is okay, what could be the problem here? how can I get this PC to play Music? the subwoofer works fine with another computer.
