The little "looks like a speaker" icon at the bottom right (the Startup Tray) should pop up with a box you can check to "Mute" the whole sound system. If should have the same effect as turning the speakers off - unless you have some friendly little elf doing a bit of arc welding inside your computer.
Is it possible that Mr might be sufficiently "trainable" to be taught to "mute" instead of power off? Surely he has sufficient "favorite rewards" that the Pavlovian method might work(?).
If that's not a possibility, a further description of the "pop pop" phenomenon might help in finding an alternative, although we all know these "domestic disagreements" must be handled quite carefully by third parties who attempt to intervene/assist.