Think about the video you would like to add audio to. Things to keep in mind when choosing the audio: 

- Style of your video

- Genre of your video

- The mood you are trying to create

- Beats per Minute that is in line with the speed of your video

- The length of your video

Browse the following ariches to find openly licenced music:

soundcloud.com/creativecommons

Here you will find mostly full-length, new electronic music.

freemusicarchive.org

Here you will find classic, organic and intrumental music.

freesound.org

Here you will find short sounds  and samples you can use to make an effect.