Here are some places where students have found audio samples, sound effects, and music.
Freesound – https://freesound.org
Large library of sound effects. Many files are Creative Commons; attribution is often required.
SoundBible – http://soundbible.com
Simple, downloadable sound effects. Many are public domain or CC-licensed.
FreeSFX – https://www.freesfx.co.uk
Free sound effects for educational use (account required).
Zapsplat – https://www.zapsplat.com
High-quality effects. The free tier requires attribution; paid removes it.
FindSounds – http://www.findsounds.com
Search engine that indexes sound effects across the web (check licenses).
BBC Sound Effects Archive – https://sound-effects.bbcrewind.co.uk
Excellent real-world sounds. Educational, non-commercial use only.
NASA Audio Collection – https://www.nasa.gov/connect/sounds
Public-domain audio recordings suitable for school projects.
Music / Background Tracks (Free with Clear Licensing)
Incompetech (Kevin MacLeod) – https://incompetech.com/music
Free to use with attribution.
Free Music Archive (FMA) – https://freemusicarchive.org
Curated music under Creative Commons licenses. Good filtering by license type.
Bensound – https://www.bensound.com
Free tracks with attribution.
Purple Planet Music – https://www.purple-planet.com
Free background music for educational use; attribution required.
CCMixter – https://ccmixter.org
Music specifically designed for remixing and media projects under CC licenses.
Note:
Students are responsible for checking license requirements and providing attribution when required. Not all “free” audio is copyright-free.