Never ever issue this command. It will begin to buzz and not shut up since there isn't a command to stop a speaker from playing a tone. You can use masking tape for about a 20dB reduction but it is freaking annoying. It's in the middle of a print, of course M112 will shut it up but omg. I gotta leave the room. I was trying to "clear" a gcode macro... some things in marlin are half-baked, as tone and macros (WX, no R) prove.
I tried M300 P0 S25000 and others and it only interupts it for a few seconds.