play DVD in 7.1 channel mode

Just bought creative lab 7.1 speaker and audigy 2 ns sound card which also support 7.1 speaker, here is the problem when I use powerDVD 5.0 to play DVD movie.

In powerDVD configuration if I set the number of speaker to 2, DVD is played smoothly at 1280x1024 resolution. However if I set it to 7 speakers, both sound and image are not smooth in the DVD play even after I lower the resolution to 800x600.

Any help is high appreciated! Thanks!