The actual Covox Speech Thing appears to use 100k R (200k 2R) resistors in a standard resistor ladder.
(this was measured here: http://www.vogons.org/viewtopic.php?f=46&t=51483 )
Designs that use other values may or may not sound the same.

While resistor ladder designs can work (the original Speech Thing sounds excellent),
it is likely better to use an actual DAC IC in your homebrew design for maximum quality.