How to merge MPEG4 with AC3 audio

Sony Vegas 11 has an annoying bug, namely that the Sony AVC encoder won't export audio. Solution: render out AVC without audio and then render out a separate AC3 audio file and merge them using MP4Box.

1. Download GPAC and install it on your computer.
2. Extract the H264 from the MPEG4 file:
   MP4Box -raw 1 attr_2_resiliance03_sonyavc.mp4
   assuming that stream 1 contains your H264.
3. Merge with AC3 audio:
   MP4Box -add attr_2_resiliance03_sonyavc_track1.h264 -add attr_2_resiliance03.ac3 merged.mp4

That's it. And it's even streamable for flash.
Update: this bug only shows up if the audio rate is higher than 192kbit/sec.