No Audio

Here’s my problem. I had purchased the DVD Night at the Roxbury a while back. I never opened it until last night and used it to test my new Computer Speaker system. I wanted to make sure the Dolby Digital input was indeed entering my speakers. Well I tried this DVD and got no audio. I then put the speakers into analog mode and I got sound. I then tried a different DVD Dolby Digital encoded and the Dolby Digital played fine. I then tried a DTS encoded DVD and it also played fine. Thinking it wasn’t my speakers or soundcard I tried Night at the Roxbury" in my set top DVD player. It also would not play the Dolby Digital audio. It did play the analog audio through my Television speaker (analog cables hooked up to the TV). I made sure to select all possible audio selections (Dolby Suround, 5.1 Dolby Surround etc.) I never got a digital signal however. Is it possible this DVD is NOT Dolby Digital encoded? Now that I write this, I’m thinking that I figured 5.1 Dolby Surround would be the same as Dolby Digital. Now I’m thinking this 5.1 Dolby Surround is actually analog. Does anyone know if this DVD is Dolby Digital encoded? I guess I should just look at the DVD or the case and look for the Dolby Digital mark.