I know there are quite a few types of audio/music playlists out in the wild. Are there any that are specifically approved as FLOSS? That is, are there are that are particularly encouraged by the FSF, like how .ogg is an encouraged audio container format?
Scai seems to have it answered in the comments. A lot of these are just plain text, so there is no proprietary license on them; use whichever you'd like. M3U seems to be a popular choice.
EDIT: polemon pointed out that some plain text formats such as ASX (XML-based) require a license for use. Watch out for things like this.
The same community that brought you Ogg, FLAC, Vorbis, and now Opus, created XSPF.
From a practical standpoint, however, there isn't many software out there, that supports it. I still tend to use it for little projects, etc. but aside from RockBox, VLC and I think Foobar, there aren't that many players that can understand
From a design standpoint,
It supports metadata, of course. Adding metadata to the individual tracks might be quite redundant, as that metadata is usually stored in the files themselves, but you might want to put info about streaming media in there, or the playlist itself, possibly. XSPF supports all that.
Personally, I've supported that playlist format on a web radio project of mine. I added it, since I was using Ogg/Vorbis, so I though it would be quite a natural addition, to make the whole thing a bit more complete. But I must say, acceptance by the audience was very minimal, most clicks were out of accident, or out of curiosity. I would suggest not relying on XSPF alone. If you have a project that deals with audio playlists, make sure you support other playlists as well...