Take care to use content that you can secure permissions for, or else you may run afoul of copyright restrictions.
There are many sources online for open source audio files — that is, music tracks that are provided for free for artists and filmmakers to use. You may need to provide credit to the musician/composer in your video. Here are a few online libraries of open source music tracks:
Please review the Guide to Clearing Rights for further information on copyright protections and restrictions.