QMediaPlaylist has a method

addMedia

with the signature

bool QMediaPlaylist::addMedia(const QMediaContent &content)

http://doc.qt.io/qt-5/qmediaplaylist.html#addMedia

And the documentation suggests for c++ that

playlist->addMedia(QUrl("http://example.com/movie1.mp4"));

should work.
http://doc.qt.io/qt-5/qmediaplaylist.html#details

BUT in PyQT5

playlist.addMedia(QUrl.fromLocalFile("some/path/music.mp3")

errors out with unexpected QUrl.

So….. you have to do

QMediaContent(QtCore.QUrl.fromLocalFile("filePath/fileName.mp3"))

in python. A bit unwieldy but PySide2 appears to have stalled from an outsiders perspective.

Initially I suspect that QUrl and QMediaContent inherited from some sort of common base class and were split off and its perhaps an untested use case of using QUrl was lost. I ended up making a bandaid in python with

MakePath = lambda x: QtM.QMediaContent(QtCore.QUrl.fromLocalFile(x))

where QtM is `from PyQt5 import QtMultimedia as QtM`