Previously I was using QuickTime Player to open the .dat file, then export to QuickTime. But once I play the final product created from iDVD, there is no audio.
That is why you need to use an MPEG-based, third-party converter. The QT "engine" only converts the video content. If you want to retain the audio, you need an MPEG-based media "engine" to bridge the gap between "muxed" content and QT. (I.e., MPEG-1 is only playback compatible with QT applications -- not convertible or editable.
Depends on what codecs you use. Export to quicktime will put the audio and video compression formats you select in an MOV file container but then iDVD has to convert whatever format you send to MPEG-2 and AIFF files which are then multiplexed to 1 GB VOB files as your "title" set. Files must be "conversion" compatible with QT. Once again, I would recommend either DV or AIC/AIFF here. These compression formats produce large files because the have high video data rates. On the other hand, because they have high video data rates, they generally produce the best quality.
