If your smartphone's too quiet, you can always buy some speakers. If you'd rather keep your budget to under a few bucks, you can make your own amplifier out of a plastic cup and two bobby pins.
In addition to the cup and pins you'll also need a razor blade (or Xacto knife) to cut into the cup. You basically cut two slots for the bobby pins, bent into an L shape, and another hole so the smartphone's audio can travel into the cup and reverberate around inside (which will amplify the sound). The bent pins will hold the cup in place, making your phone look like it's wearing a backpack.
If you want to give this project a try, hit up WonderHowTo for the full instructions.
Amp Up Your Cell Phone's Sound System with a Plastic Cup [WonderHowTo]