I'm not a legal scholar, but it seems to me that the two instances are being dealt with in a similar manner. In writing terms, inserting a portion of someone else's work into yours without crediting the source is the definition of plagiarism. I am probably missing some legal fine points, but in concept it seems like the same thing.
It's not the same at all, no legal fine points about it. Inserting an actual bit of someone else's mechanical recording is completely different legally from whether or not a composition is plagiarism. Only the recording artist is entitled to make a derivative of his own recorded work unless he grants permission for another artist to use a sample.
