I had the same question…
I tried playing a music file on my Voice Preview device and using the Developer Tools → States page to see what it was playing, but it doesn’t report the media/content information.
So I played a music file on one of my Google cast devices, which I know will show state information for the media that’s playing. The reported values were URLs for my local HA instance and a path to the local file, like this:
If you have media stored in config/www:
http://IP_ADDRESS:8123/local/media_file.mp3
If you have media stored in media:
http://IP_ADDRESS:8123/local/media/media_file.mp3
Those URLs work for both the preannounce_media_id and the media_id.