Transformation is the key word when it comes to using someone else's creation within your own artwork.
Since it's a class assignment or part of your educational program, Fair Use exceptions should apply.
Be careful using any copyrighted material in works you plan to display publicly, including any music. Also, audio recordings don't have the same rules as text and images for when they are automatically out of copyright.
When looking online for stock audio files, always double check the license or terms of use. Some sites offer free files, or you can pay a licensing fee to use it in a public work. This is called Commercial Use. Royalty-Free doesn't mean free cost, just that you don't have to pay ongoing costs to the copyright owner, just a one-time fee.