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.
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).
Hit "Start," use your keyboard space bar, or just hit the big red circle to start recording.
When you are finished hit "Stop" or hit the big red circle.
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.
Review the file
Now you can review the audio file before finishing.
Pick a license
You can also choose a license, including providing alternative attribution information in case you are not the original author.
Approve and finalize
When you are done hit "Use This" or "Close" to finalize the new sound.