Under which Creative Commons license? there are six types. All require attribution. Some allow modification, others don't. Some allow commercial use, others don't. See an attorney for further details.
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This depends on the specific terms of the specific creative commons license--there are many of them, all of which have different terms and limitations. If the creative commons restricts commercial use, then a film festival is per se a commercial use. People enter film festivals with the hope that their films will receive awards and then find a wider audience---this is a commercial objective. I am very skeptical of using creative common licenses for purposes of using songs in films---this almost never works.