When you say "AVS" are you asking about Alexa Voice Service, or the new Alexa Voice Simulator? If it's the simulator, I believe the announcement of the tool said that the audio tag doesn't work properly in the simulator. If it's the voice service (and your ssml works fine via the echo), you might be better off asking one of the voice simulator forums.
Friends, Seems that it was on our side, Since the introduction of the SSML audio tag, we receive several elements in the payload when we have a mix of TTS/mp3. The appropriate behavior is to extract all the payload elements and to play them one by one. Best, Hicham