Audio-output trailer frodo 12.2

Hi!
I have a optical digital audio output via usb plugged to mu laptop and has set it to the default output audio in xbmc settings. When i play a movie the audio plays through the usb optical to my stereo as i want but when i choose to play a trailer the audio instead comes throug the laptop speakers.
Why does not the trailer audio go throug the optical digital audio output via usb?