Sylfaen as originally designed had a full set of International Phonetic Alphabet characters, and they appear in a paper on the designers' website. But the version that was free with Windows XP — the only one I've actually used — did not include those characters. Have they ever been released in any form?
As of 2004, it hasn't. In an excerpt taken from Typophile, John Hudson (the author of the font) says this:
It has been nearly 8 years since that post, but I do doubt that they were ever released if he never had any plans for releasing them.
John Hudson did however redraw Sylfaen under a new name, "Nyala" which does have some IPA Extensions according to Microsoft.