The last step would be to weld the external audio and batery cables to the thin internal cables.
You may need to cut bigger the hole in the mouse case for the three cables to pass through.
When you can close the upper cover it´s done.
weld the + and - of the audio output to audio jack, or rca
Now hotglue the two batteries to the top of the top cover of the mouse.
Plug the batteries and the audio jack and play.
You can use 4 or 6 CR2032 batteries, button shaped. much smaller, that may fit inside the mouse.
And if you install a small speaker to the mouse you have a fully portable device :)
You can create images like the one in this picture, trying to create different wave shapes as a normal sinth would.
I will update this instructable if I get the 40106 oscilator working to make a sound depending on the brightness of a surface.