ZX81 competition

Richard claims to have produced sound on a Sinclair ZX81 (a machine with no sound hardware) at the age of 11:

When I was 11, I won 50 pounds in a competition for writing this program that made sound on a ZX81. You couldn’t make sound on a ZX81, but I played around with machine code and found some codes that retuned the TV signal so that it made this really weird noise when you turned the volume up.