Set mediaplayer volume

I use an automation that is triggered by an alarmsystem which does serveral things.

One of those should triggers alexa to say “something”.

Currently “something” is played at the last known volume setting.
I’d like to have it as loud as possible.

The problem is that it seems like it is not possible to control the volume while the echo is in standby mode. The volume buttons only appear when something is player.

Is there a way to set the volume to 100% and after “something” back so x%?