Hello you Canon enthusiasts out there, hello ERR99!
I tried the simple phototransistor->soundcard cable.
My first thought was: if the signals are to weak, you probably have to amplify them, just like the comparator LM311 does in
the RS232 blinking receiver.
The mouse uses infrared light (>780nm) led, an so the photodiode in my mouse is only sensitive to infrared light.
Well, I have several pieces old Typhoon mini LED mice operating in the visual red range. If you were living in Germany, I'd sent you one quickly.
So, bottom line is: Use only photodiodes which are sensitive to "visible" light, like the SFH 300 (420...1130nm) , because the G11 LEDs does not emit light in the infrared spectrum. So i am grounded now and have to wait until my photodiode order arrives... damned! 
Surely is the blinking receiver a better way. So I'm exciting to wait for your next try! Good job, ERR99!
Nice greetings!
