Most music is protected under international copyright law in the same way photography and illustrations are protected. It is illegal to sample bits and pieces of songs you love to use in your own audio or video works. In the same way as you must not simply grab whatever you want from the results of a Google Image search, you should never use any music you please without first establishing that you have permission to use it.
Observing the copyright of other writers, photographers, video makers, and illustrators is very, very important when you’re putting together your multimedia project. You may not use just any old photo or illustration you find using Google Image search. Similarly, you cannot simply “reblog” great stuff you find on other people’s blogs or web sites. Using other people’s work without permission is illegal. It also devalues the work professionals do, which may make your own work worth less in the future, when your work is your livelihood. That’s worth thinking about!