This article is from the comp.sys.acorn FAQ, by Paul Vigay with numerous contributions by others.
The *Speaker command does not work on new models of Acorn machines. The A300, A400, A3000, A540, A5000 and A4 all had software control of the built-in speaker. With newer machines this feature has been removed in favour of a automatic hardware cut off of the speaker when a jack is inserted into the sound socket on the machine.
However to ensure compatibility with old software the command *Speaker has been left in the OS, it merely doesn't do anything.