I am not aware of any AES device that actually does this. As AES is often used for clock, it would be highly disruptive to interrupt the AES stream simply because the audio in the stream was silent.
It would seem to me that this something that the speakers should implement (e.g. if they sleep after getting no audio for some time, they should be able to distinguish between AES with no audio vs. AES with audio), since the speaker has already implemented the logic for it's low power mode - making it dependent on no physical signal rather than no audio signal seems like an oversight.
In any case, implementing this would be quite involved and likely to cause problems under a wide variety of circumstances.
While we would be willing to consider such a feature, it would be something that would relatively low priority given the other things on our backlog and the complexity that would be involved.
If you would like us to keep track of this, it would be best to file a feature request via MIOConsole Help > New Feature Request...