I have seen some speakers and thought about this also, are you talking about the silver bullet looking type speakers that are supposed to mount to the handlebars? Wonder if they could be mounted to the downtubes, are they directional? I don't know how well speakers would work riding down the road with the way the wind whips you around and beats your ears up, at least with a half helmet, think I'll stick to earphones myself.
At their size, they are fairly directional... But, since sound disperses in air, it will not be like throwing a dart at a board, more like soft waves in a pond after a rock goes into it. Unlike subwoofers, which are considered by most, multi-directional, (which is why you hear bass before you hear treble or higher frequencies) and will create the bigger waves in a pond. At a smaller size, the frequency range will be more limited to a narrower bandwidth than a larger speaker.
They would be better on the handlebars, on the forks, would probably lose a lot of sound at moving speed, (unless you blasted them, then you run the risk of blowing them up, due to over amp'ing them) due to wind noise, road noise and the other objects in the way.
I used to build speaker boxes in my teens for people and I have installed many stereo systems. I guess I have always been interested in electronics...