Hmmm ... I don't think I've used that function in Womble. Are you using the export tool (in demultiplexed mode) to generate your AC3 and MPV files? Also, double check the "audio" tab in export to be sure that AC3 is checked. If you cannot check it, it means either the audio has been manipulated in some way other than a straight cut, or that two different types of AC3 are being mixed.

It is possible that my AC3 will not mix with an "official" AC3 as Vegas exports 5.1 AC3 files at 448 instead of the standard 384. I tried to change this but the software wouldn't let me. There are very long ways around this if this is your problem. Or, you can just downmix to a stereo WAV or Layer II file. You'll save the hassle, but lose true surround.