I believe you get no-void ply by simply selecting A-A or A-B grades.
Both are pricey furniture stuff, but it seems like there's always an extra premium placed on "marine grade".
Marine grade is supposed to be water-resistant (none of it is really waterproof) but like Gekko said, it's probably all about the same these days. None of mine have ever been in the rain for more than a few minutes and never the bare wood.
Voids are not thing! Yes, legend has it you'll get 'buzzing', but I've never seen it happen in dozens of homebrew projects with every kind of questionable grade there was -- right down to CD grade concrete form, crating crap. Can't say it won't ever happen and never has happened, but it's been the subject of considerable more stories than experience.
I think it's a 'selling point' for speaker builders. And maybe they've seen it happen. Or maybe they just wanna sell speakers. Dunno.