Whether music found online can be used for free or not depends on the copyright terms. Most music that carries the Creative Commons attribution is free for use in non-commercial projects. Check with the copyright holder for more information.
Even if a piece of music is attributed as Creative Commons, there still may be some terms that must be followed. A couple of examples of Creative Commons licenses are the CC BY-NC-ND attribution, which allows downloading and sharing non-commercially as long as credit is given and there are no alterations to the original material; and the CC BY attribution, which allows altering and sharing of the original material so long as credit is given. CC BY also allows for commercial distribution.