Electrical Removal/Dead-End of Wires for Old Intercom

Below you will see pictures of the "setup" I am wanting to remove. What I want to do, one room at a time, is remove the intercom system from throughout my house. The first room I am doing that has this system has the main box, and an 8-Track player, both still work by the way.

What I plan to do is kill the power to the room, ensure that the boxes have no power as well. Remove them from the wall, dead-end the wires using caps, and place them back in the wall before patching the holes with drywall. I found instructions here. But one of the steps, reads as:

Remove the cover of the main intercom unit using the screwdriver. Locate the two wires that supply power to the unit, usually a red wire and a black wire connected to the unit via a terminal. Use the voltmeter’s red lead to touch the red wire’s terminal while the voltmeter’s black lead is touching the black wire’s terminal to check if there is still power being supplied to the unit…Cut the intercom unit’s electrical wires using the electrical pliers. Cap the wires with the wire caps. Tuck the capped wires inside the hole on the wall where the intercom unit was previously mounted.

My question is – If I turn off the power to the room, and the power turns off the player and the voltmeter reads zero…how do I know I have safely dead-ended the wires? Additionally, does anyone have any safety tips for this kind of thing.

This is a not duplicate of this question as that one deals with the patching of the hole, while mine asks about the wiring. Two different parts of the same dilemma.