First select the tracks to export. When exporting all tracks use CTRL+A. Then set the time range. If the export starts at the beginning of the project all that needs to be set is the end. To set the end, move the Now marker to the "end" of the project and either hit the "Set thru=Now" button in the Select Module (B in the image below) or use the Edit > Select > "Thru=Now"  menu option   To change the export start is similar except position the cursor where the sele