Audio in sync in HTML output, but not in MP4 for YouTube

I'm putting short videos from a camcorder into my Captivate 7 projects. To get better sound, I'm using a microphone to record to a slide during video taping, then using Media Encoder to export to FLV without the sound track from the camcorder mic. I'm using Windows Movie Maker to trim off the first part of the video and soundtrack where I'm turning the camcorder on. The timer in MM shows where the sound starts, and I trim the soundtrack in Captivate to start at that instant.

When I publish as HTML, the soundtrack is synced correctly. But when I publish to YouTube Widescreen HD, the soundtrack is out of sync. I have a bunch of videos to publish on a deadline, and this is driving me nuts.

Edit: One more thing, when I copy just the video slide to a blank project, then publish that as a YouTube video, the audio syncs correctly. In the complete project, there's one slide with some artwork and text before the video slide.

Another edit: When I delete the first slide, the audio in the YouTube video syncs correctly. So there's something about having a slide before the video slide that messes up the soundtrack sync.

Any suggestions?