Here is a setup of the audiodescription plugin in combination with an XML playlist:
As you can see, the audidescription file is not defined in the embed code. Instead, it is defined in the playlist, separately for each entry:
<rss version="2.0" xmlns:jwplayer="http://developer.longtailvideo.com/">
<channel>
<title>Example RSS playlist with audiodescriptions</title>
<item>
<title>Coronation Street</title>
<description>This entry contains an audiodescription</description>
<jwplayer:file>/videos/7OCSON1y-393434.flv</jwplayer:file>
<jwplayer:image>/thumbs/7OCSON1y-480.jpg</jwplayer:image>
<jwplayer:audiodescription.file>/videos/iWeFNNeM-393434.mp3</jwplayer:audiodescription.file>
</item>
<item>
<title>Sintel Trailer</title>
<description>This entry does not have an audiodescription.</description>
<jwplayer:file>/videos/3XnJSIm4-364766.mp4</jwplayer:file>
<jwplayer:image>/thumbs/3XnJSIm4-480.jpg</jwplayer:image>
</item>
<item>
<title>NS Accessibility</title>
<description>This entry contains an audiodescription</description>
<jwplayer:file>/videos/AKIUnF9G-364765.mp4</jwplayer:file>
<jwplayer:image>/thumbs/AKIUnF9G-480.jpg</jwplayer:image>
<jwplayer:audiodescription.file>/videos/RCJfkBlp-393434.mp3</jwplayer:audiodescription.file>
</item>
</channel>
</rss>
The playlist is in RSS format and uses the jwplayer: extension to display the audiodescription options.