This reminds me of those little toy guitars which play little riffs when you "strum" the "strings". I, being a decent musician myself, love this add-on.
If I may ask, where did you get the sounds for the guitars? If you made them yourself, you're a rather fine guitarist. Otherwise, just to be safe, I'd recommend giving credit to wherever you got the sounds from.
In addition, I would recommend changing the "auto-fire" rate to 1.8 seconds. That way, when people hold down the left mouse button, they'll be playing riffs in-time with each other.
Also, if you ever decide to make more "types" of guitars, I would like to see one with more distorted riffs for a more rock-y feel.
One more thing. Instead of having a dedicated silent prop guitar, how about making a button that "kills" the sounds of the guitar? Maybe you could use the light key or something.
Great work on the add-on. I look forward to seeing how far this can go. I will be keeping an eye on this topic.