There are different ways to select or highlight files.

Method 1 Press and hold the Ctrl key and click any number of files with the left-button of your mouse to select or deselect.

Method 2 Press and hold the Shift key and click two files with the left-button of your mouse. This will highlight a group of files at once.

Note that if no files are selected, Audio Converter and Ripper will assume all files as selected. So if you for instance want to play/convert/rip all of the files in the list, you don't need to select any files.