Mmm, thought someone would ask that.

Well, there can't be ALL that many 'unknown, out-of-copyright and as yet unpublished' (which is what it would have to be to qualify) material left, but you could make a case for saying that as soon as a piece is recorded, or published, or performed in public, or enters the modern public domain by one means or another, it does at that moment effectively become 'collected', even if not by someone who calls themselves a Collector in the normal sense. So the term would still work. You could call them Source Songs if you prefer I suppose.

I don't like it much either, but the current situation is a mess.