<a href="http://www.casperelectronics.com/images/finishedpieces/speak-n-spell/Speak-n-Xbending/LFO%20schematic2.jpg"><img src="http://www.casperelectronics.com/images/finishedpieces/speak-n-spell/Speak-n-Xbending/_LFO%20schematic2.jpg" width="400" height="271" alt="" title=""  /></a>
Here are schematics for adding a pitch modulating LFO to your Speak&Spell. 
I also added some simple mods that work well with the LFO. One is a "HOLD" effect which grabs the sound that is playing when it is activated and loops it. The other is a "TONE" effect which generates a straight tone. 
This is a pretty rough design and I'm sure it could be cleaned up and improved, but it's simple and it does work pretty well. 