Try setting the play mark ( the blue arrow) to where you want to start. You have to have the ruler displayed ( 'options' menu) to show and set the arrow. Right click on the ruler to show.
Then temporarily add a double bar where you want to stop.
HA exports just that section.
Ahh .. I see there's another method. Select the bars you want to export. Click the " plays the selected range only" icon ( its the one with horizontal lines and arrow above, ) HA exports only the selected sound.