Anyone have any ideas why I don't have any audio on my newly authored DVD?

A bit of history on what I did first. I converted the audio from a xvid with 2 channel PCM wave file into 6 mono wave files using cool edit pro. I then converted the 6 mono wave files into a 6 channel ac3 file with sonic foundry soft encode. So far so good. I then used TMPGEnc to multiplex my m2v (which i encoded earlier) with the ac3 file I created to check for any audio problems etc. The new mpeg file plays in windvd and power dvd with 5.1 surround sound perfectly. I then authored in dvd-lab and the final dvd doesn't have any sound. I did exactly the same thing to another xvid with 2 channel stereo and it worked a treat. Can anyone tell me what I have done wrong? I can't understand how I managed to produce a similar project before.

Thanks in advance for any help.