Please bear in mind that US Law presumes(!) the existence of copyright – whether or not the author "registers" it.
Therefore, you must now do what's known in legal circles as due diligence. Start keeping a diary, starting right now: "On [Date-A], at [Time-A], I encountered [specifically this] at [this URL]. Wanting to use it, I, on [Date-B] at [Time-B], through [personal-decision-B], attempted in good faith to contact the presumed copyright owner. (Insert records of all other attempts here – showing resourcefulness and creativity as well as respect for copyright.)"
Keep equally-timely record of every encounter. Keep your diary forever.
And – if your project is in any way commercial, "engage a qualified lawyer."
Your defense – should you need one – is "innocent infringement." And,to make that stick, you need to document your every "rights-respectful, good-faith move."
"Your Honor, I did respect the possibility of copyright, I did pro-actively look for it, and here is my diary, which also records my immediate take-down the moment I was first informed."
At the very least, your video or other work product should contain a prominent visible disclosure of the exact source(s) that you used. And, if anyone should challenge you on it, you should be the one to take it down, also recording this in your diary.
"I wanna quit being a computer consultant and become a composer and arranger at age fifty-nevermind."
Logic Pro X, MacBook Pro, 88-key MIDI controller.
Just south of Chattanooga, Tennessee, USA