Amazon recently changed section 6 of their publishing guidelines in version 2013.3.
Embedding audio and video in mobi files is now possible. The description and sample code on how to do this is complete. Unfortunately it does not mention how to test your file before delivering. I tried to sideload my files onto a kindle fire. The files looked fine, but did not display any of my audio or video content. I contacted amazon support an got an interesting answer on why this is the case.
Amazon uses the old KF-7 (sic!) format for displaying embedded audio and video content.
As of to date the new KF-8 Format does not support audio or video. This means, if you sideload your ebooks to one of the Kindle fires it will find the KF-8 version of the file in the mobi package and therefore not display audio or video …
When the file is submitted to amazon. They will parse the file for audio and/or video content and deliver a specially crafted mobi7 version of this file to customers with a kindle fire. Sounds complicated? It is indeed.
So how could you test your files?
The only way to test ist with kindle previewer set to iPhone/iPad mode. This mode displays the KF-7 version.