Helmet laws in my sate exist. I have a good friend who purchased a helmet with speakers inside of the helmet..I do not know where he got it but It is worth a look. I see the cruiser style bikes with the speakers on them and while wearing a helmet, they have to turn the volume so high it is annoying to other people in traffic. That is why Mark went the other way.
I do not believe it would be difficult to adapt any other type of speakers to your bike. With enough ingenuity, anything can be accomplished. Good luck and have fun while you do it.