Problem casting to Google Next Mini

I have set up Google Assistent and Google Cast Service and can cast to the Next Mini from my mobile.

I have also set up a Lovlace UI component

type: media-control
entity: media_player.hall_speaker

And can turn it on (and here a sound when it is turned on). But when I try to play a mp3 from the /media folder using the button in the UI that lets me chose a file the play icon apears breafly but there is no audio coming out of the Nest Mini.

I can access /media using Samba and the mp3 play fine.

Any idea how I can I fix this.