Fascinating.

Some early computers, such as the Elliott 803, had a built-in loudspeaker which received a pulse every time a jump instruction was executed. This meant you could tell which part of the program was executing, or whether it was in an infinite loop, just by listening. See, and indeed listen to, http://www.survo.fi/demos/#ex88

Hah, had a lot of fun listening to those. Some of them sound a bit like some digital rf modes.