Hi @TobbeJ,
I got the same problem and managed to solve it using the following configuration:
volume_mute:
service: media_player.volume_mute
data:
entity_id: media_player.sonos
is_volume_muted: "{{ is_volume_muted }}"
And then also provide an attribute to the universal media player with the mute status:
attributes:
is_volume_muted: media_player.sonos|is_volume_muted
Hopefully this works for you as well.
Cheers!