Extending and Reducing the Selection Using the Cursor Keys
To move the start/end of a selection in the wave window to the left/right, hold down Shift and press the left/right cursor keys. To move it in bigger steps, press the Page Up/Page Down keys.
To extend a selection to the previous/next boundary in the wave window (marker or start/end of the audio file), hold down Ctrl/Cmd+Shift and press the left/right cursor keys.