Mixkit is a good place to find free video clips and audio clips to use in your multimedia projects. I wrote about it earlier this week and mentioned it on my podcast. For those who would prefer a visual overview of how to use Mixkit to find free audio an video clips, I offer the following tutorial video.
As I mentioned in the video, the licensing terms for assets on Mixkit are pretty clear. You can download videos and audio files from Mixkit to re-use and remix. You don't have to credit Mixkit, but they will appreciate it if you do.
If you're looking for more places to find free audio files to use in classroom projects, check out this list that I published on Practical Ed Tech.