We have a thing for electricity-free iPhone speakers, especially when they are DIY projects using cool old materials. Flickr user lowtechatmo posted a couple images of their "home-brew, low-tech iPhone amp made of PVC and an old bike horn."
The project looks fairly simple:
The bike horn (with it's noise maker removed) had just the right amount of threading on the end to screw into the soft plastic hole, and be flush with the inside of the PVC. It sits directly in front of the tiny mono speaker. Leaving the ends of the PVC open allows for a more expansive sound and lets more treble escape. The weight of the horn keeps the phone from tipping back.
If you happen to find an old bike horn in a thrift shop or antique mall and are wondering what to do with it, well consider your problem solved!