Sorry it took a long time to respond. Been busy.

Here is the fix that worked for me: https://www.youtube.com/watch?v=f5n6IeBCw3A

Do note that downloading dll's from the internet is dangerous.

I hade the same problem, and so does many other it seems. Are you trying to record multiple audio streams? That was the issue for me. Vegas only seems to parse the first two streams in a file. The fix was to switch out the dll that was parsing the .mp4 files in vegas (I think to qt7plug.dll?). Anywa...