Error handling if source does not support non-audible playback

This has come up on the EME List:

What is the expected behavior of MediaElementAudioSourceNode and other
source nodes if the source does not support silent playback or redirection
to a buffer?
In the case of EME, a ContentDecryptionModule may refuse to output sound
any other way but directly to the audio hardware.

I imagine it may throw an error or treat it like a silent source, but I
can't find anything definite in the Web Audio API spec about error handling
in this case.

--hans