ac3 avi movies don't play back in 5.1 on divx dvd player

hi all,

thanks for reading this and for any replies i get.

i have just bought a divx dvd player (toshiba sd-270e) so that i can stick 5 or 6 backup avi movies onto a blank dvd and free up space on my hard drive. some of the avi files say they are ac3 encoded, which as far as i understand, means that with the correct equipment, they should play back in 5.1 surround. the dvd player is hooked up to a yamaha rx-v357 home cinema receiver with a digital coax lead. when i watch original dvd’s, they are played back in 5.1 surround, but when i watch the ac3 avi backup movies, they only play back in boring old 2 channel left and right stereo.
i have also tried burning a single ac3 avi backup movie onto a dvd as a dvd movie. i used winAVIvideoconverter to create the required dvd files, and then nero to burn the files onto dvd to play back in the dvd player. this method only gives me 2 channel stereo as well.
where am i going wrong? any answers/solutions i get would be grately received!

cheers,

funkyspaceman