When editing a board, the default behavior is for vocalization buttons to generate speech using the device's text-to-speech synthesis engine. However, you can also provide your own audio for each button if desired. You can also specify a license for these sounds, so others who copy and modify your board will be able to give proper attribution.
Open the button settings window
To attach a sound to a button, hit the button to open the button settings window.
Choose audio recording
To record audio, hit the little microphone icon to open the recording tool. Note that not all devices and browsers support recording audio on the fly (the recording tool uses HTML5, not Flash).
NOTE: Keep an eye out for the camera permission popup that your browser may show. Some of them aren't terribly obvious, but the camera preview won't show up if you haven't allowed CoughDrop access to your camera.
Review and accept
After you have recorded an audio clip you can review it, select a license and assign the sound. You can also try again or abandon the recording. To finalize, hit "Use This".
Select the file
Select the file from your device that you want to provide.
Fun side note: You can also drag sound files from your device's file browser and drop them on the button you'd like to assign them to. This drag and drop feature is not supported on some browsers.
Pick a license
You can also choose a license, including providing alternative attribution information in case you are not the original author.