There are 3 errors that I found in the circuit alone. All component references are from your circuit and not the webpage schematic 1). C4 should be 47uF instead of 1uF. This is minor. 2). The connection from the positive side C1 should tie direct to R2 and not to R4. 3). You have Q3 tied directly back to 12V with no load. The LV Speaker is not a load device.
The tricky part about this is that in the original schematic the speaker forms an integral part of the oscillation network and without a proper inductive load the oscillation will not be correct. The bad part is that Multisim does not have a component that can properly simulate the load and inductive properties of the speaker, therefore it is not going to give good results due to this fact. You can put an 8 ohm resistor in series with a coil to simulate this effect (not the best but will work) but you would have to know the inductace of the coil inside the speaker.
As far as the LV Speaker. It will not work in all hardware situations. It doesn't work on mine because of my sound hardware. Nestor could explain this to you a whole lot better than I could. It is the way in which the computer speakers are controlled. Nestor explained to me that if your speakers, mixer, etc were controlled by the sound driver instead of Windows then the LV Speaker would not work. The output device has to be controlled by Windows in order for it to function. If I didn't explain this correctly then Nestor can correct anything I may have mis-stated .This is not to say yours is not working, but I thought I would point this out to you.
I hope this helps some as this is all I could find.
