As with video footage, make sure that you check what rights you have to use free resources, if your project is for commercial gain, always give accreditation to the composer. It’s only good manners!
This is not a legally qualified explanation, just a rough guide
What does “Royalty free music” mean?
You pay a one time fee, then you can use the music download without restriction
What does “Public Domain” mean?
These are completely free without any permission being required.
What does “Creative Commons” mean?
The music is completely free BUT you have to have permission from the artist or give attribution or both