Students should be learning Digital Citizenship lessons when using technology. When creating video, teach students about copyright, fair use and how to responsibly use other's creative works. This is ethical use of digital resources.
https://www.youtube.com/audiolibrary/soundeffects - includes both sound effects and music files. Some don't even require attribution- look for the drop-down filters at the top of the search screen (requires google login)
https://www.soundsnap.com/ (requires free account- recommend that teachers do so and then have students select what they want for teacher to download (listening does not require an account). Sounds are categorized and easy to navigate.
https://freesound.org/ (requires free account- recommend that teachers do so and then have students select what they want for teacher to download (listening does not require an account).
Soundtracks
http://www.grsites.com/archive/sounds/ (free sound effects: *** NOTE: when opening the page, there is a yellow box that links to Paid content. Scroll below it for the copyright free resources)