To select a part of the audio file you have to click on its desired beginning point on the waveform and then while holding
down the left mouse button move the pointer to selection's end point.
Alternatively, if you know the start and end time or duration of the selection, enter the start time, end time or duration in the appropriate text boxes on the lower side of the screen.
You can also select the whole audio file by clicking on "Select"->" Select All" of the "Edit" menu.
To clear the selection, just click on a random point of the waveform or click on "Select"->" Select None" of the "Edit" menu.
Select Left / Right from Cursor
To select a part of the audio file that is on the left or right of the cursor (i.e. current position) click on "Select"->" Select Left from Cursor" or "Select Right from Cursor" of the "Edit" menu.
Then enter the desired length of the selection.
Select Track Start / End to Cursor
To select from the track start to the current position or to select from the current position till the track end,
select "Select"->" Track Start to Cursor" or "Track End to Cursor" from the "Edit" menu accordingly.
Select Till End of Previous Label / Till Start of Next Label
To select till the end of the previous label or till the start of the next label select
"Till End of Previous Label" or "Till Start of Next Label" from the "Edit"->" Select" menu.