problem with audio....

for seem reason the audio on certain Quicktime movies and certain streams on the internet are being played only thru my USB LOGITECH HEADSET??? I checked the system prefs. and the output is set for my ex. speakers. iTunes and other applications play thru my ex. speakers. I had this prob. before, with iMovie as well, movies would play audio only through my HEADSET not my ex. speakers. The problem went away, then just appeared again!! I sometimes get a warning window when closing certain apps. " new settings have been made, do you want to keep these settings.. ? " Why does this pop up when I haven't changed anyhting??? I tried repairing permissions and still the same problem.