Just had the idea riding in this morning, but I'm sure someone will have had it before. It wouldn't be that hard to set yourself up with a microphone in the helmet and a decent amplified speaker on the bike somewhere. (Maybe a 'press to talk' button on the handlebars so you don't go all Darth Vader on everyone.) If you could get it nice and clear - not like the PAs on most train platforms, for example - then you'd have lots more options than just the horn and the finger for communicating with cars, other bikes and pedestrians. Has it been done? Can it? And then, apart from the odd wolf-whistle, what might you find yourself saying? Keep it clean as far as possible... A couple of people on the ride in this morning would have got a touch of "They have these newfangled things called indicators now - ever heard of them?" A lot more people than just me would hear my muttered-in-the-helmet "Don't you even contemplate thinking about pulling out in front of me" comments.