A lot of my friends can tell you I've tried every route to get music on my motorcycles in the past. On my Nighthawk I bought tweeters and a small amp that I kept in a tank bag. On my ST I bought marine grade speakers and a marine grade stereo and installed the stereo in the trunk. The ST was by far easier to do all that and run the wires without people noticing due to all the fairing plastics. The nighthawk was a pain in the but to do that with, not to mention the splices would eventually vibrate loose. If you choose the speaker route I suggest getting rumble road speakers or the speakers that someone else noted in this thread as they're made for what you want to do, the connections don't fall apart due to vibration, and they're either pre-amplified speakers or they come with a super miniture amp which is really easy to connect to the bike in an unoticeable spot. If you try your own way you'll end up more frustrated then anything
When I bought my america I decided to give up on the speaker thing as the ready made speaker kits for motorcycles are way over priced, although from what I've seen from friends whom have them, they're awsome if you want to spend the money. So I bought a motocomm (similar to autocomm and starcomm) and just hook my xm or mp3 player into that and hit the road. Much easier to do and a lot less spendy. Not to mention you get added features like being able to hook your cell or two way radio to it as well so that you have bike to bike communication.
Another option is they sell ear-muff speakers at kohls every winter for about $5. I think a friend of mine even has an extra set..will check on that. He uses them and his nano and doesn't have any problems. Then again his bike isn't nearly as loud as mine either. My uncle tried the same on his harley and didn't have any luck. Kinda a toss up, but its a cheap thing to try.
Hope this helps a bit.