Setup server and player as normal, create a play list containing the “Off Air” message on repeat with an InSession playlist that plays a message on repeat  An example “The legislature is in session but the audio is currently disabled. Please stand by for resumption of the session” – Create a show for each session that plays the InSession playlist

On the player, enable the scheduler – Enable the audio input on the Sources tab – Check the “Disable When Silence Detected” and “Prioritize Above Scheduler” options – Restart player

In pulse audio, make sure the line in is using the correct audio source

When a session is not scheduled, the player will play the DPL, which will give an off air message. When a scheduled session occurs, it will start playing the special InSession message until the audio on the mixer is turned on. When that audio is detected, the player should switch to from the scheduler to the line-in, and play the line-in audio until silence is detected again (ie. the mixer output is turned off), in which case it will switch back to the scheduler, which will again play the InSession message.

There is a 2-4 second delay in switching in order to prevent rapid switching. If a pause lasts longer than that, it will switch to the InSession message.

https://www.openbroadcaster.com/case-studies/legislative-broadcasting/