Using unlicensed audio in your video's is against YouTube's terms and is getting
many people's YouTube's accounts shutdown.
Often without warning.
Take one of the biggest kids' YouTube channels "Vladscrazyshow" for instance...
They had OVER 8.5 million subscribers, over 1 billion views and making a lot in
ad revenue, then in November 2017 their channel got terminated for infringements with
YouTube's guidelines.
It's safe to say, YouTube doesn't mess around with copyright infringements.
And if you want to actually sell videos to clients, you MUST have licensed audio
as this can also affect not just you but also your clients.
So it's absolutely essential you have royalty-free licensed audio if you are
using video or plan to use video in your business.