How Do I Navigate From File to File On My 819?

In the main menu, select “NAVIGATOR” with Forward or Back. Press M to Confirm and Enter Navigator. Select a file or folder you want to play with Forward or Back and Press PLAY to play that file.

Alternatively, when in music, voice, movie, or e-book mode, you can press the M button to enter the submenu which will allow you to select folders and files of that type (music folders in music mode, voice folders in voice mode, etc.)