Save/restore volume on (slimproto) Mediaplayer

Before making certain announcements, I’d like to set the volume of the media player to a certain value. However, afterwards the old value should be restored.

From what I have seen, there should be a volume_level entity but it doesn’t exist for me.

Does the slimproto media player not support this attribute? If so, why not?

Which other way do I have to achieve my goal?

I know nothing about this particular media player but can you go to developer tools, states, find your media player, have it playing something and then post here what’s shown regarding attributes etc?