DVD standard would be 48,000Khz (PCM or MPEG-1 layer 2) for an NTSC encoded disc. I think before you were having the problem of outputing individual rather than elemetarystreams (i.e a seperate file for audio was created). You ended up creating a system file (both video and audio combined) which your Ulead application preferred.
However, it's more than likely to be the sampling frequency that's throwing up the prompt - I believe this was around 32,000Khz in your other post?
what format IS the audio you're trying to author to ntsc dvd?
you can have:
Up to 8 audio tracks containing DD (Dolby Digital/AC3), DTS, PCM(uncompressed audio), MPEG-1 Layer2. One audio track must have DD or PCM Audio.