Any stereo that runs off batteries (e.g. portable iPod docks, dollarama pocket speakers) can be easily made to run off your scooters DC power, it'll just need a DC power converter (<$10-50) to match the voltages. Making the entire system function easily (change settings) and look good is the challenge, but fortunately you only have to do it once. You'll need the space to hide all the components and route the wires, cleanly mount the speakers, and put some thought into accessibility (can you reach the volume knob hidden in your fairings/rear or will you need to relocate it. Need a cable to connect your iPod or will you bluetooth pair it OR leave a USB stick plugged into an AUX usb port).
For the most cost effective solution, you're looking at retrofitting a car stereo. They already run off 12-14V DC and speakers are plentiful. Car stereos/decks can be had for cheap on kijiji (<$50), along with their speakers (I assume you'll only need 2 in front unless you want surround sound
). Connect the decks power to your battery with an inline fuse, a switch to turn the system on and off, and the speakers to the decks front-right and front-left channels. Mount the volume knob and aux cable somewhere accessible (unless you have room to mount the deck in your dash as well).
With wind and traffic noise, and the open environment (the fact car interiors are enclosed spaces helps their speakers sound great; ever notice in BestBUy and FutureShop the car spearks on display are in a smaller room?), I wouldn't bother investing on a quality amplifier (e.g. >$200 car deck or standalone DC amplifier) and speakers. If there's a chance you'll EVER leave your scooter in the rain, get marine speakers (or spray the exposed parts of your speakers with a flexible permanent non-conductive coating) and waterproof covers for anything else exposed (e.g. your in dash deck). If it's a hidden component: thicker bags with zip ties and silicone sealed openings are fine, just make sure everything inside is safe and free from being punctured.
Car decks can draw up to 50W per channel. Two speakers = 2 channels => 100W at full volume. For a 12-14V system (your scooter) that's 7-8.5 Amps. To find out if this will be a problem find out how much power your stator can continuously produce, subtract the power of all your accessories (even the bike itself to power the headlights, dash, engine, to recharge the battery, and your vest) and how ever many watts or amps you have left over is how much you have to play with. If that's only 4 amps, don't turn your system up to full; or find places to save power like replacing your stock headlights with more efficient HIDs or LEDs. Upgrading your stator is another option (exactly equivalent to putting a stronger alternator in your car to make more electrical power for your heated seats and hot/cold cup holders).
