Contact the author. Without a certificate, he is the one who can lift the claim.
Disputing a claim with Youtube is never the best way to lift it. Especially in this case where there is no license certificate. With nothing to back your dispute, Youtube may think you’re being “dishonest” and this may hurt your channel. You normally want to contact the third party (usually AdRev or Audiam) who will lift the claim swiftly when presented with a license certificate. In this case though, going to the author directly is the way to go.
Authors need to think twice before offering an AdRev registered track for the Free File of the Month. Offering a track to anyone who wishes to use it and yet trying to monetize illegal use, just does not compute.
It creates complications, and a lot of additional work for the author, since they would be responsible to clear the claims to those who downloaded the free file… all of them.
More importantly, distributing a file freely (even though temporary) is considered by Youtube to be incompatible with ContentID. Creative Commons music was taken out of the system for that reason. If many Free File users dispute the claim with Youtube, saying the file was distributed for free, you run the risk of having your track removed from ContentID, or worse your whole portfolio, or worse the whole microstock industry.