Have you heard about the cup trick? You can amplify your built-in-iPhone-speaker's volume by resting it in a dry glass cup. It's called a "passive speaker," and it works kind of the same way that the hollow body of a guitar or violin helps to amplify the sound made by the strings. Of course, leaving your iPhone in a glass isn't the most gorgeous solution for amplifying sound without extra power.
If you want a more permanent solution, give this DIY project a try. Renee from Red Bird Blue
built this iPhone dock from just wood, making this a completely power-free amplifier that can be endlessly customized to suit your style. We love the natural-plus-neon look, but you could sand, stain and paint yours to your tastes.
This wooden dock turns up the volume the same way guitars and glasses do. The sound from the iPhone's built-in speakers gets sent through channels in the wood and out through two large holes at the front.
You'll need a few "more than basic" woodshop tools, as Renee mentions. But this project is well worth the effort for anyone who wants a little more volume without extra cords and the satisfaction of your own self-made product. Leave one in your bathroom for shower karaoke, and make another for the patio outside!
(Images: Red Bird Blue; plans via Woodwork City)