I have Alexa voice commands working with Roon, not sure if anyone has tried this before so I am reporting in case it is useful. It is bit of a hack.
I recently got a Fire HD 8 and the show mode stand. I had already setup a Harmony hub and used the Deep Harmony extension for use with a harmony companion remote. The deep harmony extension emulates Roku device so volume, play pause, skip forward, rewind and few other commands work with the remote,
I then linked the Harmony hub to Alexa but be sure to use something other than ‘Roon’ as the friendly name (Alexa confuses it with ‘room’). Alexa does not like ‘Music’ either as it will start playing Amazon HD so choose something like ‘Jazz’, ‘Tunes’ etc.
Voice commands such as play, pause, volume up, down , skip forward work. I have setup Harmony hub to control the volume of my DAC.