Transport Menu: Looping
From Audacity Development Manual
These commands enable you to control loop play.
Loop On/Off L
Toggles loop play to be on or off.
Clear Loop
Clears the looping region in the Timeline.
Set Loop to Selection
Sets the looping region in the Timeline to be the same as the current selection.
Set Loop In
Sets the start point of the looping region in the Timeline.
Set Loop Out
Sets the end point of the looping region in the Timeline.