Hi doc,
Well if I want to author a DVD using multiple Video streams AND multiple Audio AND multiple Subs I always choose Apples DVD Studio Pro which is the developed Maestro cause they bought the rights -But this is done on a Macintosh.
In your case you should take a closer look at Maestro the latest PC avaiable Version, it's still very good and to me it seems the origin core where DVDlab is based on. That's why I dont understand why multiple Audio isn't supported in DVDlab
It's just a economical decision I think to give a main attraction to a following Pro version.
Or, .. do the DVDlab way as explained in the link above and try to put multiple subs in your DVDlab project.
I'm not that "Sub" fanatic .. you know