I'm guessing that you WANTED a simple 1 Video track and 1 Audio track - yes? At the moment it would apppear you are doing a 5.1 surround sound project? You wanted to do this?

OK, I'd "Save As" as a new copy "TestVeg" and start again on this TestVeg. But this time remove ALL the 5.1 sound mapping. DO Keep your original VEG, but remove any 5.1 options within this TestVeg.

You may also want to investigate reading & updating your VP8a to VP8b - it is free! - read any of the white papers on this newer "b" release and see if there HAS been an issue with 5.1 - yeah?

Interesting . . . .

Grazie