CS70 wrote:If you have the YT video, that's pretty good evidence of your copyright, as neither you nor the infringer can alter the video timestamp.
As of the cassette - it's odd that it's deteriorated in 20 years... unless the conditions where it was kept were awful. Not sure if the US copyright office has any specific way to handle perishable specimens of any type (photos are another possible example among many).. but I guess it's just to ask them.
This response is for you, and mof. Do you think the yt video would help in court if the copyright tape ends up sounding so terrible they can't hear it? I could try to talk to the copyright office again. Last time I called they put me on hold for over an hour, so I hung up.
As much as the music is recognizable, it should. The thing about copyright claims and counterclaims, is that it is a litigation. It's not often that it's a open and closed case (but it could be, in which case you may be offered a settlement). It will be theirs lawyer team to evaluate, and yours to evaluate and give you advice.
To the extent that the music is clearly recognizable, a video stored and timestamped by independent third-party which is not easy to tamper with, is pretty strong evidence - but a case must still be built and litigated.
Your copyright office registration should be sufficient to allow you to file a claim (I guess you're in the US as you talk of a Copyright Office with capitals).
If things are more nuanced (like a progression is similar, or lyrics are of the same tone but not very similar) , you may expect a legal battle to proof or disproof that your video contains the same material.
In any case, the best you can do if you want to pursue the claim (which means, if the "stolen" song is having enough commercial success to claim damages, a cut of the proceedings and a royalty credit such to justify the expenses) is to find and consult a music copyright specialized attorney (or two or three). He would be able to look at the specific case and evidence and give you an opinion better than any forum in the world.