It works flawlessly but only when media_player.alexa is in playing or paused state.
Problem is in standby state (active most of the time) - setting volume has no effect and text is spoken at the volume set previously which is usually too quiet.
There needs to be an active queue (playing or not) for the volume controls to work. This is a limitation of the unofficial API that the Alexa website users.
apparently I will have to try to play some very short mp3 file to bring media player out of standby mode and only then adjust the volume.
UPDATE: No luck of bringing media player out of standby mode by playing some local audio file. Alexa just says Sorry, text to speech can only be called with media player alexa_tts service.
I have (almost) fix the problem inverting the set volume with annunce, in the sequence i put first the annunce them the set voume, in some case the “bling blong” alexa annunce sound is cliped but the result in non bad.
now I have a new problem to fix.
annunce is asynchronous, sequence do not wait until the text to speech is done
probably the only solution is find and set one appropriate waiting time (in the example 3 seconds), not very easy with data_template text without prdefinited time.