The iPhone is already a powerful smartphone, more than capable of playing videos and your music. Pair it with a decent speaker system, and you will get a better experience out of the device. Trobla can also help. It is a wooden amplifier that does not require a power source to work. It looks good and has a simple, sustainable design.
The developers have used a core and special chamber to provide you with a sharper sound. The Trobla can be adapted to handle various smartphones thanks to its detachable parts. The amplifier is available in maple and walnut versions. You can find it on Kickstarter.