Step 1: Download MP3s
Just get some sound into your computer and then into your MP3 player.
Suggested sounds would be those of a motorcycle, but why not a Jetsons car sound, The Electric Slide, or the Back to the Future theme song?
Step 2: Dig up some speakers
I am using self-powered, battery operated speakers of the kind that were used with portable CD players.
They have a small amp built-in, and run off four AA-size batteries.
Step 3: Install the speakers
Hidden is best for the gag of an electric cycle making gasoline cycle sounds.
Under the seat or inside the hollowed-out gas tank can both work well.
Attach the speakers with zip-ties, screws, glue, or whatever works for you. I like zip-ties, because they hold tight, but are cheap and can be easily removed.
Remount the gas tank or side panels to cover the speakers when you are done.
Make sure the wire between the two speakers reaches.
If the speakers have a power button, make sure it is still accessable after mounting, or modify the power switch to relocate on the handlebars or another convenient location.
Step 4: Mount the MP3 player
Use hook and loop fastener, double-sided tape, bongo-ties, or something similar. That way, you can remove the MP3 player for security.
Alternatively, it could be mounted inside the gas tank or another secure location, if you have a wire remote for it.
I used fancy rubber bands to pin down this iPod.
Plug in the cable from the speakers to the headphone jack, making sure there is enough slack to be able to steer the motorcycle properly without pulling the cord loose.