So Polymorph general rules say:
If you take on a battle form with a polymorph spell, the special statistics can be adjusted only by circumstance bonuses, status bonuses, and penalties. Unless otherwise noted, the battle form prevents you from casting spells, speaking, and using most manipulate actions that require hands. (If there’s doubt about whether you can use an action, the GM decides.) Your gear is absorbed into you; the constant abilities of your gear still function, but you can’t activate any items.
Note the use of term "battle form". It is not capitalised, which according to page 17 means it is not a special term.
This would mean Humanoid Form prevents you from speaking or casting spells. On the other hand, it does not mention "battle form" in there, unlike say Dragon Form which morphs you into "a Large dragon battle form". It also specifically allows the use of manipulate actions:
While in this form, you gain the dragon trait. You have hands in this battle form and can take manipulate actions. You can Dismiss the spell.
But this still says nothing about speaking (which dragons can normally do) or casting spells (also something they can do normally). So at least that is clear I guess?
My guess is, "battle form" should be capitalised and it should be a specific term separate from simple morph effects. And yet it isn't. Are all Polymorph effects limited?======================================================================
On a side note, as a big fan of Transmuters, specifically for polymorph effects on allies and terrain, and not as dispensers of Haste/Slow/Baleful Polymorph. This is yet another blow. Dragon Form was the one spell that looked really fun, even despite the abysmal duration.
Sure, the idea is that character should be mauling the enemy with their new shiny claws and teeth and tail... But what if situation changes and they need to shout a warning or cast a spell? It's just another strike against the viability of Transmuters.