I see a number of people asking about how to get free music for their videos. In order to use music in a video that you intend on monetizing and making money off of, you have to have the rights to monetize the video. That can mean one of two things.
One, the person who made it gave you permission to use it in your video. If you know someone that makes music, whether it be from the forum or in real life, you can ask them. Make sure they say you can though!
If you don't want to ask permission, you can use any royalty-free music. Royalty-free means that you're allowed to use the music in content that you intend on monetizing, under some contingencies. Normally, they require you to mention them somehow, whether it be in the credits of a video, in your description, or whatever, give them credit. One popular site with royalty-free music is:
http://www.incompetech.com
If you want more, you can look up "royalty free music" on google, and should find something. If you know of any other reliable sources that you'd like to share, please link it down below for the use of others.
I hope this clears things up for everybody! Thanks for reading!
EDIT: Also feel free to check out Th3StuffShow's music, which he allows free use of as well. He just asks that you credit him. Here's his channel for music:
http://www.youtube.com/user/th3musicstuff