Quezi

Gramophone (photo by Taras Kalapun - CC-BY)

If you use a commercial recording as the soundtrack for a video that you upload to YouTube, you risk the removal of the video or the stripping of the soundtrack if YouTube determines that your audio track is not properly licensed. By choosing appropriately-licensed music you can have a soundtrack that’s free, legal and guilt-free.

A great source of legal music is Jamendo. Most of their music is avilable under one of the Creative Commons licenses. The most useful of these is the Creative Commons Attribution License (abbreviated to CC-BY). You can use CC-BY music freely, even commercially (e.g. on a website that includes advertisements).

You are allowed to share and remix any CC-BY work, provided you attribute the work (provide credit to its creator). You can browse hundreds of CC-BY-licensed albums from this Jamendo page. Here’s a page from which you can access Jamendo music under other CC licenses.

The Internet Archive also has a great collection of Open Source Music, mostly licensed under the various Creative Commons licenses.

For your YouTube video, you might also want sound effects. The Freesound Project has a great collection of downloadable sound effects. The ones I checked used the Creative Commons Sampling Plus license, which lets you freely use them to create commercial or noncommercial mashups, provided you credit the creator.

You could also check out ccmixter, although most of their music is available for non-commercial use only.

Kevin MacLeod has released as large collection of his music under CC-BY.