mp3-Files I drop don't have the mimeType 'audio/mpeg'
From the list of mime types, it looks like Windoze is saying, "Launch the default shell application to try to open this mime type", which, depending on your associations, could be Windoze Media Player or some other third-party app.

I would guess that if you selected the "text/uri-list" ("uri" meaning "Universal Resource Identifier"), it would contain not the mp3 contents, but the address(es) of the selected file(s) on your PC as pseudo-URLs (e.g. file://c:/Music/myfile.mp3"). I am guessing that the QVariant would be a QStringList type.

If all of your drags and drops are internal to your app, I would think you could make up your own mime type string: "application/myapp-songs" and use that when putting your Song metatypes into the drag and drop mechanism. It wouldn't have any meaning outside your app (or another of your apps that could serve as a drop target), but if what you want is to move things around within your app, then it should be fine.