The ISD1820 voice recording IC is a simple, relatively low quality voice recorder. There are lots of details on how to tweak this chip <a href="http://casperelectronics.com/wp-admin/post.php?post=2177&action=edit">here</a>. 
After some experimentation I found that the chip can also be used as a fun voltage controlled filter. See the schematics below followed by a little circuit theory. 
Here's the basic sampler circuit. <a href="http://casperelectronics.com/wp-content/uploads/Simple-Sampler.png"><img src="http://casperelectronics.com/wp-content/uploads/Simple-Sampler-460x326.png" alt="Simple Sampler" width="460" height="326" class="alignnone size-large wp-image-2216" /></a>

By activating the FEEDTHROUGH switch, the sound at the input will be fed directly to the output. BUT along the way it passes through the ISD1820s built in filter. The filter cut off level is set with the pitch knob. Under "normal" settings the filtering effect is pretty soft. But with the expanded pitch setting I have added in the schematic it is possible to get a wide filter sweep.

In the schematic below I've removed all of the extra hardware thats not needed if you want to JUST make a filter. 
<a href="http://casperelectronics.com/wp-content/uploads/VCF.png"><img src="http://casperelectronics.com/wp-content/uploads/VCF-460x285.png" alt="VCF" width="460" height="285" class="alignnone size-large wp-image-2227" /></a>
The only extra feature here is the FEEDBACK/RESONANCE setting. The resonance behaviour will change depending on which speaker output is fed back into the mic in. Try both and see which you prefer. 