It worked, now it doesn't, and you don't know what you changed.... The chances of us being able to help with this limited information are pretty slim.
Does it do this on all content (audio files and movie files) or just movie files? All movie files or just a certain filetype? All files of that filetype or just some? From your hard drive or streamed from the inet? Or both? What version of Quicktime? What OS? What soundcard does your machine have?
In general:
You must have installed a program, or made a settings change, perhaps without knowing it, which has altered your system.
You can:
Look back and see what programs you have installed recently,
See whether you have upgraded QT or your soundcard drivers,
Check the soundcard settings in detail
Remove and reinstall Quicktime