You need to check the setup menu on the AVR-1907, and assign the Optical 1 digital input to the DVD input. This basically tells the receiver where to look for sound when you flip it to DVD. You should find the option in the menu under "Digital Input Assignment".
Also, don't forget to check your speaker setup and tell the receiver you don't have a center channel connected. Otherwise, you won't hear any voices when listening to 5.1 sources!