Well, I can only tell you what I do (had the same problem as you when first starting to burn VCDs with Nero). I convert all my files first with an external converter. And since doing so, I've never had it again that the pictures and voice were out of balance.
I use this free converter, which is absolutely great:
http://www.erightsoft.net/S5E01.html
By the way, that's now the second time that I have to edit a post of yours regarding personal information; please make sure it is THE LAST TIME !!!!