For this, the older and more simple the monitor, the better.
I did this in the early 90's with an old monochrome screen. No sleep mode. No multisync. No self-diagnostics to shut it down when I disconnected both deflection coils, and hooked them to the left and right channels of a stereo.
Of course, that wasn't very smart. I wasn't changing the brightness according to how fast it was scanning. So when there was no audio, the electron gun remained centered, focusing sufficient energy to light the whole screen into a single point. It instantly burned out the phosphor there, leaving a permanently dark spot, and a smoke trail rising up the inside of the screen.
At the time I wasn't sure how to make an automatic brightness circuit, so I ended up reconnecting the vertical coil to the internal circuit. That way the gun was scanning, at least a bit, at all times. Then I set the brightness so that it wouldn't cause any damage when there was no horizontal signal.