converting ePub3 with embedded audio for Kindle Fire?

I have a series of ePubs with many short embedded audio clips (all .mp3) that work well in iBooks on iOS & Mac. Now I'm looking into making them work on Kindle Fire.

I successfully used the KindleGen utility in Terminal to convert to .mobi, and the images & text were OK on the device, but no sound controls were visible.

I then tried KindleUnPack v0.80 to split the mobi file into mobi7 (.mobi) & mobi8 (.azw3) versions. The .azw3 version still shows no audio controls.

The mobi7 version shows speaker icons where the sounds are, but tapping them gives the message "Unable to Play Audio File - The selected audio content could not be played. Please try downloading the content again."

In the unzipped version of the mobi7 that KindleUnpack made, I see an 'Images' folder with the images, but no sign of the sound files having been copied over.

Is there a step I'm missing? Any way to do this? The audio has tags like this:

<audio controls="controls" preload="preload" src="media/German_4.02.mp3"></audio>

Thanks!