Hi guitarix upstream maintainer here.
Please downgrade the severity of this bug. The files in question don't be part of the distributed package, so there is no serious reason to mark guitarix "unfit for release". Even if the [debian/copyright] file may be wrong for those files, that haven't any relation to the distributed package, as they are only part of the source, and be in no relation to the distributed binary. If it helps, I could mark the files in question as CC-BY-1.0 so that the copyright file is correct. However, this is by far not a serious bug, so please, handle it.