The audio controller is your personal sound board in Flipside. It will remember the audio files you've added to it and their settings between sessions on a per-project basis, so you can always jump back in where you left off last time.
You can find the audio controller at the bottom of the the Audio menu.
Note: You can also find the audio controller under in the Props menu in the Show Tools category under the All Props tab.
Audio clips that are added to the audio controller have a number of options you can use:
The clips that you select can all be played together by clicking on the Play selected button.
There are also the following dropdown options for configuring how an audio clip should play:
Audio that is played through either the audio browser or audio controller while recording will automatically play back the same when you watch the recording.
Note: Spatialized audio sources are coming in a future update.