Toggle Cycle Preview ⌃C
One of the many commands that have no particular documentation.
In the Project Audio Browser window there are playback controls. Play, Cycle, and Volume. The Cycle button is the named ‘Cycle Preview’ button.
When the Cycle is enabled the selected audio region will loop on playback.
Cycle button: Loops playback of the selected audio file or region. Use in combination with the Play button.