Soundsystem for bicycle
I have seen numerous people riding down the path with a sound system on their bicycle. Without exception they use the simplest and most portable solution: a bluetooth speaker. This meets all of the requirements of outdoor usage:


Self Contained
Waterproof
Long Battery Life
No wires to run 
Lightweight and portable
Inexpensive




Tie it to your bike rack and pair it with your phone. I've done it myself once, but be warned, you will need a louder one outside (especially at a critical mass ride). So you may have to invest in one of the larger models.