AWS.MechanicalTurk.XMLParseError headaches with an audio file as an EmbeddedBinary

I’m working on an app that leverages Amazon’s Mechanical Turk as a transcription service. In the HIT, I want an embedded recording that the worker will play and transcribe.

To do this, I planned to use an EmbeddedBinary element in the QuestionForm XML description. I tried this:

However, submitting this in my HIT creation threw a

validation error. After a good deal of trial and error, I came up with:

(There’s a SubType element in there now.) What the documentation doesn’t seem to tell you is that the “optional” SubType element isn’t optional at all when embedding audio files!