Are you sure you don't have a "digital coaxial" input on that receiver? I'd be very surprised if you didn't.
Use the "digital out" RCA output on the DVD player, set it to output in "PCM", connect it to a "digital coaxial" input on your receiver, and configure it to use that input for your audio.
That should get you Dolby Digital 5.1 at least.
If you're using the red/white output you're definitely only getting two channels and if you're lucky, you might get some fake surround using DPL.
In any case, most movies don't make much use of the surround channels. They are used mainly for special effects, ambiance, etc, and it' not unusual to not hear them for long periods of time.