For some reason, WordPress doesn’t generate the enclosure for me even when I post the link straight up. It doesn’t do so either, when I use the ‘Add media’ function, although I’m not sure if it’s even supposed to.
Luckily, it’s possible to hand-craft the enclosure for a post for which no enclosure has been created. Though laborious, it works. The “enclosure” field’s value should consist of three lines:
Full URL to the file
Length of the file in bytes
Content-Type of the file (like “audio/x-vorbis+ogg”)