Smartphone speakers aren't very useful in a quiet room, and they're basically pointless with any ambient noise. Use an empty vase or jug instead to amplify and direct the noise.
Similar to the alarm clock in a cup trick, but on a larger scale, this can boost your phone's volume just enough to use it in a pinch if you're entertaining friends or if you want to listen to music in the shower. If your vessel of choice can lie flat, you can also try turning it on its side to direct the sound in one direction. It works surprisingly well if you need a volume boost in a hurry.