1. Start at the main note-choosing screen...
  2. Tap the settings icon in the upper right to open the settings pane.
  3. Tap the audio speaker icon in the upper right to toggle the sound off and on. The icon will turn red when the sound is off and green when it is on.

The sound setting for the app will override the mute switch on your device. In other words, even if your device has the mute switch engaged, if the sound icon is green, you will hear sound.