Is any caching of audio files done by Google or Amazon as audio content is being delivered to Google Home or Amazon Echo? I've read that developers do set up their own CDN for these purposes, but it's not clear if any caching happens during delivery or if audio is delivered directly to user devices from the publisher.
To my knowledge no, they do not cache responses as a large point of the assistant apps is to provide customised experiences to each user.
The one thing I might not be so confident on however is the inline SSML <audio> tag. There seems to be no information on the Alexa documentation or the Actions on Google documentation on this about caching, so I'd assume they don't.