If you need a little kick to your iPhone's volume, this is a free way to magnify the sound of the small speakers.
Step 1: Step 0: Place the IPhone in the Bowl.
Yep, it's that easy. Just put your iPhone right side up in the bowl. The speakers will be facing down, meaning the sound waves will bounce off the bottom and sides of the bowl. This creates an effect not unlike the horn of an old-timey gramophone.
If you're looking to play music for a room full of people, you'll need to buy a pair of proper speakers, but if you just want to listen to This American Life in the next room this is a great, free way to get some additional use out of an object you probably already own.
Step 2: Appendix: Other Possible Vessels
Obviously, the same effect can be created by just about any curved, open-topped vessel.