I have no problem using audio tags to play mp3 files with Alexa Actionable notifications.
When testing I found that the text submitted to script.activate_alexa_actionable_notification should not be surrounded by quotes. In other words,
text: <audio src=‘https://etc.mp3’/>
works, but fails if surrounded by quotes.
My endpoints are on S3. Further errors will occur if the mp3 was not converted as described in Speech Synthesis Markup Language (SSML) Reference | Alexa Skills Kit.
You can create a second implementation of the skill and remove the .ask(‘’) from the return of class LaunchRequestHandler. The second skill just plays the text it gets from HA without waiting for a reply. Clone script.activate_alexa_actionable_notification, change the script name, and put in the new skill ID. Now you can play any SSML (of course, I haven’t tested every possible tag).
Let me know of problems or better solutions.