here's a thought...

could arduino with a wire receive a longwave radio signal? then send it out to a speaker on 2 more analogue pins? (external transistor)

You mean like a 200kHz long wave signal?

or

http://www.smeter.net/stations/hf-code-practice.php

morse code receiver... or sender :P

any useful low frequency wave i guess, the most expensive part would be the 400 feet of wire you'd need for good reception lol.... what about that signal that's used to set watches?... or some other band we can steal info from..... pager?