Help with digital audio on my mac mini

i have a macmini running mac osx 10.6.8 I have installed the supplemental update for 10.6.8 that is supposed to fix this problem but does nothiing. i Have a behringer usb audio unit that has an optical out. whenvever i use this i get cracking and popping on the audio, if i switch to hdmi on tv, im fine. but no 5.1 speakers. it does this on Vlc as well as quicktime and itunes. i've tried switching optical cables and outputs and same popping noises. please help if possible. also i have used this optical usb unit for a few months with no problems, and some videos work as long as they are not 5.1 older films....