I've just built a steam engine sound generator, and it works! My plan is to mount it on the bike, with a decent-size speaker, so that I sound like a steam locomotive as I'm going along.
One of the problems with a bike (or any other electric vehicle) is that it doesn't make much noise. People don't step into the road in front of a car because they can hear it coming; they can't hear a bike, so what they do is, step into the road, then look. But since a bike is usually close to the edge of the road, they just stepped into the bike, without warning. It's not usually fatal, but it certainly can be painful, to both parties.
But no-one will step into the path of a steam locomotive.
There's also a train whistle; I can use that as my "look out" warning when the situation calls for a bit more than a "ting" on my bell.
If you want one, it's the Velleman kit number MK134 "Steam engine sound generator" - you can get them on Ebay.