Of course you have the right to “dispute” this claim, since the free download is basically the same as buying a “Music Standard License” - so you do have a valid license for the track. The only problem being that you won’t find a license certificate for this track in your ‘downloads’ section as you would normally do for a track you bought.
Yet I think there are ways to show that you have a valid license:
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When you downloaded the free file, there was a PDF included in the (zipped) download containing the license certificate details for the track. If you still have this PDF, you could use this to verify that you have a valid license (even though it was free…).
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If you don’t have this PDF anymore, you could try to contact the author of the track via his/her profile page. Send along the details regarding your video (like the video URL…) and see if the author can release the claim on your video.
I’d be very much interested to hear if you could solve the issue, since I had a free audio file this month and of course wouldn’t want people who downloaded the track to run into problems.