There are some reputable free music library sites online where you can access legal music files to add to youtube videos. They may not come close to Mozart, but for a thank you credit in your video you can legally add background music of your choosing. The hard part is wading though a LOT of music clips to find the right sound to complement your video.

YouTube will also automatically mute copyrighted or restricted material if it's within its database.

There are royalty-free music services out there, but that doesn't mean it's free to use, and you could get into hot water if you didn't pay.

Your best bet is to contact the artist and make 100% sure that you are allowed to use their music. I have seen cases where an artist has created a piece a piece of music, then someone else has taken the music and re-released it under creative commons.

We also have a lot of "musicians" who produce music that is clearly a derivative work of someone else. Just be sure you are dealing with the IP owner and not someone who is playing the system.

Try where possible to provide credit.

Mack.

Well you could try to ask Mozart for permission, but he is no longer a composer... he's now decomposing.

keyplyr ;)

Compworld, the area of music rights is one of most complex legal areas in the entertainment business, and there are people and companies who do nothing but clear such rights. Even if a composer is... er uh... no longer composing, there are performance rights to be considered, as well as all sorts of usage rights. The gold standard book in the field is called "This Business of Music", and if you're going to use music commercially, you might want to check it out.

If you get a used copy, make sure the edition is up-to-date enough to cover issues encountered with sourcing and using on the internet.

PS: It's worth noting that if you picked Mozart in the first place, you may well not be happy with library music. Most library music is intended to be generic, to work with material within defined stereotypes, so it tends to have a cliché-like quality to it, and that characteristic can bring down a whole video. When I've had to produce music cheaply, I've looked for talented music students with keyboards and software (or stringed instruments, or even harmonica, if it's appropriate) and I've had them create music with some distinction. You have to be careful they're not playing someone else's melodies, of course, but at least you'll have some control... and they'll get credits and samples.

PPS: It would also be legal to have a student (or whomever) render the Mozart on a keyboard setup. The issue you encountered with YouTube, I assume, was the recording and performance rights, not the music copyright itself.

It's actually nice to know that YouTube is monitoring this.

How about getting in touch with a local school/college/uni and asking if they could play/record some music for you? ;)