This adapter was created by "dreamblaster" and is documented here:
http://www.vogons.org/viewtopic.php?f=46&t=51483

This thread also includes the investigation into RE'ing the original Covox Speech Thing,
where it was determined the original Speech Thing uses a resistor ladder made up of
100k ohm resistors.  There is also no low-pass filter in the adapter dongle itself,
but rather is implemented in the amplified speaker.